I go through Watertown on the way to my parents (DeKalb Jct. right on the way to Potsdam) and did some racing (in the 90s) and riding but unfortunately not right in the Watertown area but close enough for you to take a day trip with some buddies and enjoy some new terrain.
The first place that came to mind is in Osceola, NY. I tried to look it up on Google and I think this is the address, Barnes Corners Ski, Hiking & Mountain Bike Trails Rte 177 Tug Hill Plateau Barnes Corners, NY 13626
315/376-3521. If you like well laid out and fun XC stuff it’s really worth looking to going here. What I remember about the place is that the people who owned and put the races on wanted everybody to have a good time and by doing so they put tons of work into the trials and always had food and drinks at the end of the day. They also, back then, let people stay on site and camp for free which I’m not sure if they do anymore but it’s worth checking into.
Other areas to look at, areas that you probably already know about, that are a bit further away (but since most times when I visit these areas I go through Watertown and are worth mentioning) are in the heart of the Adirondacks. Lake Placid has some great trails in and around the town, Whiteface Mt. use to have some really good trials but I’m not sure if they maintain them anymore, usually when I ride there it’s in the fall, I’ll ask permission to ride the mountain, and I ride the whole mountain for free. I use to ride at Cranberry Lake on Bear Mt. but I’m not sure that you can ride there legally anymore.
This probably doesn’t help too much, but when I think of Watertown, it’s because of these areas. Good Luck and if you’re ever down in the Finger Lakes region let me know, we’ve got some really great riding here!!