You SAY a princess party, but it SOUNDS like a halloween party to me. I am not sure what a pricess party is, but I envision pink and tiaras.
We have a set of climbers like at the Children's Museum (the vinyl covered foam blocks) and set that up in the living room when our halloween party got sort of rained out or it was too cold last year. Kids of all ages crawled all over them while the adults stood around the perimeter talking. The kids were laughing and screaming and falling on the blocks, and while simple, you would have thought it was the greatest party ever for them. I am not sure why it was so successful but it was.
I also set up a stage with a black curtain for picture taking, then I posted the photos online for the parents to download and print if they wanted to. The kids had fun posing. You could use props and they would like that too, I would imagine.
Now if this were a princess theme, maybe you could still do photos - different backdrop like pink for example, and they could play dressup? I have seen the boas at Michaels for cheap - that could be fun for them to play with and take pictures together. And you could add maybe some glitter and gloves, anything you have for dressup, maybe some tulle for tutus, tiaras, maybe a few stuffed animal props.
If you don't have the climbers maybe you could make a pillow pit in the middle of the floor for crawling and playing.
If outdoors, you can still do the photo thing, and maybe also create a safe obsticle course with straw bales (more of the halloween theme) and do apple bobbing (with just a little water).
Face painting is sure to be a hit and works for either the halloween or princess theme - either hire someone or ask a neice or nehphew to come and man the station for the kids. Balloons are always fun too.
Have fun and good luck!