Hi, Janice!
There is no shortage of wonderful items for princesses to collect!
Probably the "hottest" item going right now are the Disney Vinylmations. They come in different themes (park collections, Muppets, sports teams, countries, etc.) You will see the Vinylmations displayed in most gift shops.
You can choose from "Mystery" Vinylmations where it is a surprise which one you get or there are some that have a clear box and you can see exactly which one you are getting.
My kids love to collect, trade, and share their collections. Trading can be done right at the register. You will see a box with which ones they have to trade, or there are more "Mystery Boxes" that you can randomly trade your Vinylmation in hopes of getting a special one you are looking for.
Disney Mom Erin is a HUGE collector of these fun little characters. She was featured on the Disney Blog
HERE. You can read about her obsession, I mean collection, of her Mickey-shaped Vinyl-pals!
Another big item that continues to be a hot collectible is the Disney Pin. Pin Collecting is huge. You can purchase pins or starter kits and trade with Cast Members in the parks (they are happy to trade - this is a great way to find some unique pins!) Pins are themed from everything to characters to princesses to holidays and more!
Whatever your princesses fancy, they will find something fun to collect! Have fun!