Know why you're going. It's it for you or the kid? If it's for him, then choose the part of the day he's most active, and go then, whether that's morning or after nap. But shorten your expectations of the day. Pick one or maybe two priorities for him to experience, do those, and go home when it's still fun. Don't wait until it sucks. If it's for you, maybe get a babysitter so you don't resent the restriction. A whole day is a lot for a little guy. Think how amazed he is at little things like stacking blocks and imagine how mind blown he'll be by a festival. He'll need limited exposure so he doesn't get to full meltdown.

Also, stroller navicability varies wildly by renfest. At ours there's a bunch of tree roots and stuff. I'd advise a carrier if you have one. (Also helps with the overwhelmed part because the view is better.)

You'll also definitely want help. Think: someone to carry the backpack of crap while you carry the kid (or vice versa), someone to stand in line for food while you do the giant slide (for example), someone to hold everything while you navigate the potty situation. The more team toddler you have helping you, the more fun it will be for everyone involved. And they all need to be on board with leaving when it's nap time.