When I was aroudn that age, my parents had a sleep-over for me at a hotel. We had joining rooms, and us kids were allowed to stay up as late as we wanted. My parent's ended up entertaining too in the room next to us by having a few friends over while they watched us go crazy next door. Plus, we got to walk around the hotel a littlebit (I remember alot), and it was a fairly safe enviroment. We played in the pool most of the time. We had a cake, presents, etc...but the best part was knowing we had an entire hotel room for ourselves. It was like being queen of the castle. :) (I also remember the room being a poolside room, and with a little kitchen like fridge, and a living room, etc. It wasn't just the basic room, it was more like a suite.
You could also try Pump-it-up. It's a place in Ham lake that is all those moonwalks, and they are super fun. Don't konw if 8 yr olds would still like that sort of thing,..but even parents have fun there. They do Pizza and all the party supplies too and you get the whole place to yourself. Good luck.