For track days, check out the schedules at
High Plains Raceway
Pike Peak International Raceway
Pueblo Motorsports Park
I've ridden motorcycles on the Pueblo track (top speed was 174 mph down the straight!), and I've been in a car at PPIR -- both are really fun tracks. Pueblo is smaller and narrower with tighter turns; PPIR is more open with a long straight and a huge sweeping banked turn, but the infield can be pretty tight, depending on how they set up the road course.
I've never been to HPR, but I have friends who have both ridden motorcycles and driven cars there who say HPR is da' bomb!