Depends how much time you have--are you planning a special event that lasts a few hours or a 30 minute party? Are you planning a specific holiday party, or all the first grade parties? I'd check Pinterest for crafts and decorating ideas.
I am shamelessly stealing ideas from some of my kids' classes, so you can thank their teachers and room parents for these.
For Halloween they set up stations with themed activities, most of them educational. So at one station the kids are measuring pumpkins (using a string and blocks) and graphing their sizes, at another playing Halloween bingo, one is for snacks, one is writing names or Halloween words with glitter glue, and another is following directions to make spider hats like these:
(Kit by Lyndsay Riches)
In the spring they do a "beach party" outside where the kids all bring a towel, eat their lunch outside picnic style, and have parentun stations with water balloon tosses, face painting, beach ball games, snacks, etc.
During the winter holidays they build and decorate Gingerbread houses using graham crackers. IIRC they use washed out milk boxes from the cafeteria as the base for their houses.
