We'll be there Friday, too. Sounds like we'll have good weather!
Bring cash. Most shops welcome Master Card and Lady Visa, but cash is handy for food, tips, and other small purchases. The food is, as always at these type of things, not inexpensive. Some of it is pretty good though.
Sunscreen is not a bad idea. Comfortable shoes are a good thing--the cobblestones aren't forgiving of thin soles. Travel light if you can.
It's a *big* place and everything can start to look alike after a while. If you're planning on splitting up your party, designate a meeting place and hang onto your maps.
Traffic can get frustrating on 249. My advice is to get there early and linger late, watch the fireworks, make it a full day. There are also plenty of, shall we say, drivers who've imbibed. A friend's daughter just had a major accident last weekend when she was rear-ended by a drunk driver leaving the festival. So drive carefully and watch out for the idiots.
Gotta agree with the recommendations for Ded Bob and the Birds of Prey show. The 'mud show' is always entertaining. Cast in Bronze will be there. Istanpitta is a fantastic group that performs very authentic period music. The jousting is cheesy fun. The crafting demonstrations can be interesting to watch (glassblowing, blacksmithing, etc.)
The best tip is, as they used to say, 'lift up your cares.' Get into the spirit, and you'll have fun. Attending in costume can be a blast if you're so inclined. Hope you have a great time!
(I can't believe TRF has been going for 35 years...I remember when it was just starting. Wow.)