Nothing that you would mind if it got covered in mud, finger paint or anything else. If there's a picture day, you can dress her in something nicer, otherwise play clothes are the way to go. My daughter's daycare and now preschool outfits were primarily a mix of Carters, Osh Kosh, Baby Gap, BT Kids and on-sale Hanna Andersson.
Closed-toe shoes are a requirement at many preschools to avoid stubbed toes on the playground.
If your DD likes dresses, make sure she has on tights or bloomers or something.
Plan to send a spare change of clothes to keep in her cubby at school.