Lyons Classic Pinball is *THE* place. But they are closed Mon-Wed.
There is a new place up the road in Louisville called Tilt. Have not yet been there. Same goes for M-W.
In Denver (only about 30min if you avoid rush hours) there is 1up Downtown and 1up Colfax. I much prefer the Colfax location, a little bit cleaner, better for pinball. Both 1up locations are good, they are barcades and crowds get interesting at both. They are open 7d/week.
If you're here past Wednesday, then LCP is a no-brainer. 15min north of Boulder.