Paintball
The Sacramento Valley has had a burgeoning paintball scene for quite some time, most exemplified in the well-known professional team, Rich Telford's Sacramento XSV (which has played on the NPPL circuit for years).
Although there are plenty of fields around here, I've personally visited two, Davis Paintball Center in town and Midway Paintball Extreme on the north side of Vacaville (a 15 minute drive west via Interstate 80 and Midway Road).
Davis Paintball Center (which is where I first tried out the game) is operated by one of XSV's team members, Micah McGlocklin. Built on the site of a former landfill, several different fields are offered (large "castle" field, smaller field with a small one-room building in the middle, a NPPL-regulation airball field), and games are held on a near-constant basis on weekends (open for walk-ons 9-4 Saturday and Sunday).
Midway, out in Solano County, has a slightly different set of fields (airball, hyperball, a "Western"-themed field, and one other that escapes me at the moment), and is a bit more organized in terms of when the walk-on games occur. I personally prefer the terrain there, which appears to be a little more well-maintained, though the dry grass and brush at Davis Paintball Center may be well gone by the time I check it out again. Midway does have similar hours to Davis, though some times in the year, they will offer Friday night sessions as well.
Both offer rental packages in the $30-40 range, with players who provide their own equipment getting in for $15 (not including refills of CO2 or high pressure air). And for those with large groups, renting out an entire field of play is possible at any time.
More information:
http://www.davispaintball.com
http://www.midwaypaintball.com/
For those interested in paintball equipment, I've been to several different paintball shops in the region. The two that come to mind are:
Rage Paintball in West Sacramento, the closest to Davis: http://www.rage-paintball.com/
Predator Paintball in North Highlands on Watt Avenue, about 35 minutes from town: http://www.predator-paintball.com/
