PI for sure! My wife and I started going to WDW when we were 28 and we've gone every year since. We even joined the DVC and now we have two little kids who are freaks like we are.
Anyway, PI is awesome. When we were w/o children, we used to go to PI every single night as it was part of our package so, why not? Adventurers Club looked strange to us so we avoided it the first few nights. MISTAKE!! After we finally decided to check it out we've been hooked ever since and can't wait to get back. We even do a special night w/o the kids (who get the FAiry Godmothers) and go to dinner at Cal Grill then off to PI for Adventurers Club.
To tell you the truth, once you've seen the Comedy Club a couple times, it gets old. It's basically the same thing every time even though it's improv.
The Rock and Roll bar is really cool. Can't remember the name of it but it is just down the hill from the Adventurers Club on the same side of the street. Sometimes you have to go upstairs to get in and other times you come in at ground level and climb the stairs from inside. I believe it's got a Shark on the sign out front, not sure though. The music is great with a live band playing all the great songs from the 70's, 80's, 90's and some 2k songs also.
We also lived at Epcot back then. We would just stroll the World Showcase sipping a beer and looking at all the culture. We also ate like gorillas

Best to diet a little before you leave.
Back then we also spent a lot of time at MGM since some of the best thrill rides are located there. We love Rockin Roller Coaster with the Aerosmith theme and also Tower of Terror. Both are great. We also just love the theme and feel to MGM. We did all the hokey attractions also and loved them all. One thing that has become a tradition is the 3D Muppet Vision which is still a hoot and my kids now insist on it.
We like MK back then but not as much as the others. AK was not open back then but when it did open, we went and loved it. The river ride in the China section of AK is great. If it's hot, don't even worry about a poncho. It will feel good and you'll dry off soon enough. You will get wet though so don't think you might escape it. Dinosaur (use to be called Countdown to Extinction) at AK is also fun as is the 3D show It's Tough to be a Bug.
It's a lot easier to do the parks w/o kids and also good experience. When you one day return with kids you'll be an expert and know exactly what to do.
Have fun and enjoy. No matter what, try everything you can. You'll enjoy almost everything Disney has to offer.