I’m glad to help you meet everyone that you can! This is going to be pretty thorough and long winded, but I’m all about meeting characters and I’m excited that you are too.

DISNEYLAND
For starters, you can actually ask if certain characters will be out! There is an information booth on the left of the Walt/Mickey statue with a list of ride wait times. There’s always a cast member there who will gladly tell you if a character can be found. It’s a super helpful resource. They won’t always tell you what time, or where, but they will at least tell you if they can be found at all.

Princesses
The best place to meet them is in Fantasy Fair, back by the entrance to Toon Town. There you can meet Ariel, Belle, Aurora, Snow White, Cinderella, Tiana, Jasmine, Mulan and Pocahontas. 3 are out at a time and they rotate in ~30 minute intervals, with a different trio each time. You’d have to visit 2 or 3 times to meet all of them. Poca is the rarest one to meet, but I’ve seen her in there twice.

Rapunzel has the best meet and greet of all the princesses in my opinion. You can only meet her in Fantasy land, to the right of the Pinocchio ride at her dedicated spot. It’s worth the wait. The room is beautiful and sometimes Flynn is in there with her. If he’s there, ask him if he’ll show you her tiara (he has it in his satchel!).

You can also meet princesses near the castle. Snow White, Cinderella (almost always with the Fairy Godmother), Ariel, Aurora and Belle can be found throughout the day at the Wishing Well to the right of the castle or the walkway going around the left side. I’ve also met Perla with Cinderella there as well. I also recently found the Beast with Belle there. It was the day of the re-release, so it he might still out as promo for that.

Fab 5 
Liberty Square is the best place to meet any of the Fab 5: Mickey, Minnie, Pluto, Goofy and on the rare occasion, Donald. I have actually only seen Donald during the holiday seasons (Halloween and Xmas). Chip and Dale are also in Liberty Square on the regular.

You can also meet most of the Fab 5 in Toon Town. Pluto and Goofy typically rotate out on the right side of Toon Town while Minnie and Mickey can be found in their houses. Usually the lines are outta control. However! You can meet 3 different Mickeys at his house- it all depends on what door you go through at the end. You can meet Classic, Steamboat, or Sorcerer versions of Mickey. Sometimes Minnie is at her house in a pink dress instead of the normal red one. She stands on the porch, so her line is usually longer.

Winnie the Pooh
Pooh Bear and his friends have their own meet and greet over in Critter County. There I’ve seen Tigger, Eeyore and obviously Pooh. I’ve never seen Piglet over there, but I’ve heard that sometimes he’s there.

Pixies
Pixie Hollow is a pretty cool place to just be in. You will always meet Tink, but the other fairies vary throughout the day. All of them have been found there: Rosetta, Fawn, Iridessa, Silvermist, Vidia (who cracks me up) and Terrence. It’s the only place you can meet Tink.

Others
As far as other characters you can meet, you can really only count on a few. The regulars are Mary Poppins and Bert and Alice and Mad Hatter. They run around Fantasy Land, the castle, and Matterhorn. You can, however, sometimes find Peter Pan in the same areas.

The Red Queen and White Rabbit sometimes make appearances, often in Liberty Square but walking around in Fantasy Land. The Queen is out more than Rabbit. Last week, Hatter was running around with the Queen AND both Tweedles so that was exciting!

Pinocchio and Gepetto can also be found in Liberty Square on occasion. Sometimes together, sometimes individually. Cruella and Hook are out as well, but I believe now (in the off-peak season), the chances of them being out are slim. Same applies to the Evil Queen, who I’ve only seen out once at the Wishing Well, and it was during the summer.

Aladdin and Jasmine are out pretty regularly in a designated spot in Adventure Land, between the exit of Tiki and exit of Jungle Cruise. Sometimes they are together, sometimes individually. The Genie was only there during Halloween. Sad face.

In Tomorrowland, you can find 2 Storm Troopers on patrol. They roam the whole area, but spend a good amount of time by the Space Mountain entrance.

In Critter County, you can sometimes find Brer Fox and Brer Bear. I’ve only seen them in peak season so I wouldn’t expect to see them this time of year.

I’ve never met a character in Frontier Land. When I as in the park in a Jessee shirt, a CM told me she was out by Thunder Mountain. I didn’t meet her that day and that’s the only time I’ve heard anyone was over there.

Depending on when you are going, you can actually meet some random characters between January 20-29 in honor of the Chinese New Year. Mulan, Mushu and some others (I don’t know who else they will bring out) will be in front of Small World.

If you want to meet an awesome variety and some weird characters, try to eat at Goofy’s Kitchen or Breakfast in the Park. At the BitP, I’ve seen Rafiki, Max, Red Queen and some other characters. I’m pretty sure they vary, but any character dining is a cool experience. They come to your table and you get some one-on-one time.  

CALIFORNIA ADVENTURE
There aren’t as many meet and greet opportunities in CA, but the ones there are usually pretty cool.

Fab 5
The only to meet Mickey, Minnie Pluto, Goofy, Donald and Chip and Dale is near the entrance to It’s Tough to be a Bug. They are always wearing some fun, outdoorsy outfits and hats and I like environment of the photo op. They rotate throughout the day.

You can also meet Mickey regularly at the gazebo on Paradise Pier. I’ve seen him in both Classic and Sailor outfits.

I have met Aviator Minnie out by the plane in Condor Flats (my favorite of the Minnies). I’ve only seen her there once, but she miiiight make appearances as promo for a new mini-show they are introducing with her and a singing group. This is just an assumption, so take it for what it is.

If you watch the Disney Dance Crew show in the back lot, you can see a Street Mickey who break-dances on stage. It’s hilarious! I’ve just been told that you can meet him sometimes, too!

Princesses
You will only meet Princesses if you dine at Ariel’s Grotto. It’s another character dining experience (and just as expensive as the rest), but Belle, Cinderella, Snow White and Aurora all come to your table. You meet Ariel as you enter for an awesome photo op. You won’t find them anywhere else.

Pixar Characters
You can always meet Woody, Buzz and Jesse over across from the Toy Story attraction. Sometimes one or two are together, but more often they are there individually. They rotate throughout the day. You can sometimes meet a Green Army Man running around, too.

At the end of the Wilderness Explorer attraction across from the Grizzly River Run entrance, you can meet Dug and Carl from UP. It’s the only place you’ll find them.

Sometimes you can spot Mr. Incredible in the backlot, too. I’ve only ever seen him near the downtown facade, across the “road” from the Aladdin show.

Others
There are a few other random characters running around, pretty much only in the Hollywood Studio portion of the park. On a rare occasion you can find Max over by Tower of Terror. I have randomly seen Pluto over there as well.

Lilo and Stitch are sometimes out, usually near the Animation Academy. Stitch is more often there by himself.

You HAVE to meet Duffy! He’s probably my favorite fuzzy meet and greet. He’s super soft and likes to jump up and down. He has a dedicated spot on the left side of the Paradise Pier bridge, across from Ariel’s Grotto and is there throughout the day.

You can’t meet them, but you can see Phineas and Ferb in their street show. You can also see Mike, Sully, Boo and a few other monsters from Monsters, Inc. in a backlot show.

DOWNTOWN DISNEY
You can actually meet characters in the Disneyland Hotel! Anyone is allowed in the lobby and on the grounds. Character sets are between either 11am-2pm or 12pm-2pm (I can’t remember). I’ve seen Baloo, some of the Fab 5, princesses and a few others there before. I have no idea who is normally out or where, but it is another meet and greet opportunity.