Ideally, you should ask your baby girl if there is something specific she likes. By this age most kids are articulate enough to share their feelings and communicate their likes and dislikes. If you don't hear anything from her, then you can try a bunch of themes. These could be a Dr. Zeuss party, Big Bird theme, angry birds theme, curious george theme, humpty-dumpty theme, candy land birthday theme, art and painting theme, treasure hunt, and even a circus theme. If your budget permits, you can even add a Photo Booth and a activities like paintball or pin the donkey or a colouring contest. Have something for the parents too, perhaps a corner for them to relax and engage with kids at at the same time.
I usually would ask my gal on what theme she wants. If she wants to be princess then I will decorate it accordingly, ask her friends to wear gowns n boys to be princes or pirates? My gal wanted peppa pig theme n we had her dressed in peppa pig dress, father rented a peppa pig mascot, many peppa pig deco. Friends wearing all the peppa pig's animal friends clothings like zebra shirt etc. interesting n fun.
Try the Superhero theme kids will enjoy that