DL does not have a lot of rare characters. There are characters that you saw in WDW that will not be in DL (Splash characters, the Country Bears, etc). But for the past month or so, DL has been working on getting rare characters out.
The Princesses can be found in the Fantasy Faire, located in the Fantasyland theater near the entrance to Fantasyland. This is the only place where you can see the princesses for a meet and greet. There are long lines throughout the entire day, so the best thing would be to get there first thing if you want to see them.
Town Square is a great place to find characters. There are always some out. In the morning you'll usually see Chip and Dale, Goofy, Donald, Minnie, and Mickey (they will be in their YOAMD outfits). You'll also find Cruella, Mary Poppins and Bert, and now some new ones like Jane, Turk, Timon, and Br'er Fox there.
Ariel, in mermaid form, is at her grotto (near the entrance to Tomorrowland) throughout the day.
Pooh and his friends are in their set spot in Critter Country, right outside the Pooh ride and the exit/entrance to Splash.
Mickey/Sorcerer Mickey is located in his house in Toontown. Minnie, Donald, Goofy, and Pluto can all be found there (Toontown) throughout the day as well (and here they will be in their regular outfits).
Alice and the Mad Hatter (my favorite characters. They are absolutely amazing at DL, definitely find them!) are everywhere! They are sometimes out in front of the Mickey floral in the morning with the band, dancing and having fun. You can also find them out and about in Fantasyland, from the tea cups, to a walk way by the Matterhorn, to the old motorboat area. They also play musical chairs on Mainstreet. I haven't done this yet, but it looks like a lot of fun (check out Briberry's videos on youtube...she has a ton of these two). The Queen of Hearts, Tweedle Dee and Dum, the white rabbit, Hook, Smee, Peter, Wendy, Mary Poppins, and/or Bert will join them...not all, but a combination of these.
I haven't seen much of Peter and Wendy. They're good friends with Alice and Hatter, so you might find them with them. These two seem to be everywhere, too. I've seen Peter in Fantasyland.
The Snow Queen can be found in Fantasyland.
Buzz is in Tomorrowland.
Woody, Jesse, and Bullseye are in Frontierland, either out front of the Golden Horseshoe, or on the path by Thunder that goes to Fantasyland.
Jack Sparrow can be found only on TSI/Pirates Lair.
Mmm, that's all I can think of now.
