Tons of riding places around. One of the easiest is the VDR track in Berthoud. Just head south on I-25 until you get to the Berthoud Exit (RT56) You have to get to the frontage road on the east side of I-25 just south of the exit and you go back under the highway to the track. They cost about 10bucks a day and are open every day.
Another track is in Milliken. They call them selves three rivers. Just mind Milliken on the map and go through (east) out the other side. Look for Weld County road 19 and head south. They are on the right a couple of miles down.
In Ft. Collin there is a Forest Service Office. Go there and they can give you maps of the area's west of Ft. Collins for those area where Motorcycles are legal. I take a street legal trail bike out flowers road into the Pingree Park area. Tons of trails up there.
One of the best trail area is just south west of Denver. Its called Rampart Range. Get the detail from one of the dealers up there.
If you're wanting trails, west of you off hwy 14 is Killpecker creek. It's just west of Red Feather lakes. Farther west you have Grizzly/Helena and Hahns Peak. And you can't go wrong with Rampart Range.