Count on Rain! at least one day or riding and one night of camping. The first year we all made fun of our friend who hucked a rain coat along with him all week – then Thursday morning came and the rain started. He was the only one laughing then (to his everlasting credit he bought a round of JD shots to warm us)!
I used a relatively clean garbage bag out of a park trash can and it was a life saver. Plan on bringing a nice new one from home and rubber band it under your seat, OR a small compressible rain/wind breaker. It is more the chill on cooler days that gets me – bring wind protection for your trunk whatever it is. A cheap rear fender guard that easily mounts is worth it. Just put it out on days rain is expected if nothing else. Great advice to find a friendly bar/cafe/park/church/anything and let the heavy stuff blow past if at all possible.
Make sure your tent is good for it if you don’t like rivers running through your things at 2am. I also pack my tent in a mesh laundry bag in the morning so if it’s wet it gets dry and not musty.
Overall, when it happens you’ll realize it’s not that big of a deal, and won’t be the thing you remember most about the week! Good luck!