I used to have a "theme park outfit" of sorts. This was in as much as I'd wear the most practical clothing I owned. For example, I'd never wear jeans, and always wear some kind of combats/shorts which would dry out more quickly after a water ride. After a while I couldn't be arsed with the effort of making sure that something specific was washed and ready to go for a theme park day, and now tend to just wear my normal "weekend clothes". That, for the most part, involves jeans and a t-shirt/vest top. I've even stopped bothering with my "theme park bag" (small rucksack) and now just take my usual handbag.
To be honest though, most of my non-work clothes that aren't jeans are practical outdoor type stuff anyway (that includes my handbag). When you've spend most of your life doing stuff like canoeing, camping, or canoe-camping then that tends to be what happens.
My one rule is that I would never wear a park/ride t-shirt to that park. Just like I wouldn't wear a band t-shirt to one of their gigs either. I used to when I was a kid, but now its just embarrassing.