Dude, I want to go to frozen dead guy days!
Seriously, though, when I hear festival I think boho. Angie did a blog post about boho light
https://youlookfab.com/2017/08.....boho-lite/
if you're not into full blown boho, it's got some good ideas.
I also like Kathie's suggestion of maybe doing a little bit of thematic dressing. I saw that sort of thing a lot in Louisiana, where people wore Mardi Grad stuff during the parades, crawfish themed accessories during the "mudbug madness" festival; the "juneteenth" festival saw a lot of tribal African outfits and craftsmanship. I saw a lot of traditional garb at the Chickasaw Nation annual gathering (which is pretty much a festival with a wee bit of tribe politics/business thrown in). Simple Americana attire (think preppy, boho lite, western inspired, etc) would also work well for festivals and outdoor events IMO.
I'm with you. At 48, I don't really want to dress as a "sexy fairy" or whatever for festivals or any other reason. Ultimately, I would suggest comfort comfort comfort above all else; lots of walking, eating, shopping at the vendors, face painting, etc can get tiring, so stay comfortable!