The Gunks are a great place to ride, but they can get a bit crowded on summer weekends. Below are a few other places to ride in the area.

Jockey Hill is a nice single track riding area near Kingston.
You can find more info at: FatsInTheCats.com

Wallkill Valley Rail Trail. This is an old rail line converted to a trail that runs from Wallkill to Rosendale.
Here's a link to more info: Wallkill Valley Rail Trail

D & H Rail Trail. This rail-trail runs from about two miles south of High Falls to Hurley.
Here's a link to more info: D & H Canal/Rail Trail
Here's some info on where to park around High Falls and trail conditions: D & H Rail Trail Conditions

Vernooy Kill Falls. This trail fearures a challenging climb over a rough jeep track to some technical single-track. Not for the faint of heart.
Here's a report from a GUMBA advance scouting team: Vernoy Kill report
If you google for Vernoy Kill Mountain Bike you'll probably find a few other reviews of the trail. I was gratified to find one that said this trail was only for "strong" mountain bikers, since I found the trail to the falls pretty darn steep.

Fats in the Cats, a local mountain biking club maintains a list of several single track areas around Kingston and Poughkeepsie.
Go visit their website for more details: www.fatsinthecats.com. Click on the tab labeled Trails at the top of their page.

The Renagades Mountain Biking Club, a new local mountain biking club is actively building trails in Lippman Park in the town of Warwarsing. They will be sponsoring a couple of races this year, and a Halloween ride at Lippman Park. They have a map of most of the trails at Lippman, and are working on marking all the trails.
Go visit their website for more details: www.renegadesmtb.com.

The Westchester Mountain Biking Association, a biking club further south, maintains a list of single track areas around Westchester.
Go visit their website for more details: www.wmba.org. Click on the link labeled Maps on their main page.