Well, it's not going to get cold at all, not at 75/80 degrees. And it seems that you really, really want to wear leggings, don't you? But I'm not sure that they are appropriate for a bonfire because they may be too warm. Step outside of your comfy zone & wear shorts or a skirt, or even a sundress with a little sweater if you think you'll be cool.
If you feel you just really need to wear the leggings, wear them with a short skirt and cute tank.
Wear some jeans with a T or a nice blouse, it gets windy in big open spaces to, so bring something to cover up even if there's a big fire in the middle. Oh, make sure that you don't wear flammable clothing, like polyester and stuff.