Really depends on his thing...

If he's active: gymnastics at Barcroft or YMCA or Apex Gymnastics or JW Tumbles. Sports at Little Gym or Great Play or Jumping Jack or Sport Rock. Pool party at any of the pools in NoVa. Trampolines at Flight or Rebounderz. Ice Skating at Kettler, Ashburn ice house, Tysons mall, and I think Reston has an iceskating thing too, we got invited, I didn't go, so I don't recall.

If he's into games: laser tag at UltraZone, Bazooka, Dulles Sportsplex.

If he's into _anything_ under the sun, honestly it's an industry and there will be people who do it. Painting. Science. Legos. Pottery. We got invited to tennis. Horses. The only ones we don't knw anything about are the girly arts ones, because neither of my boys can sit thru any of these and we just send our regrets.

Flight Trampoline

My son went to a Ninja obstacle course party for an 8 yo at a crossfit gym a few months ago and now desperately wants the same for his birthday. It was in Sterling or Herndon.

Cub Run

Children's Science Center: http://childsci.org

See a play: http://www.fairfaxfamilyfun.com/kid-friendly-theatre

What is he interested in?

The current go-to in our area is laser tag. Ultrazone.

...for an 8 year old boy. Any suggestions?