Well, off the top of my head, in and around FtC there is:

City park (few ball fields, open grassy areas)
Martinez park (same as above, plus more gravel-areas and some tennis courts)
Horsetooth park has lots of spots from long grass/gravel to some rockier stuff. Lots of good jumping potential here (I do a lot of mountain biking here).
Up poudre canyon a bit there are a couple bike trails that have gravel/small rock areas, not real wide open but enough to bounce around a good bit.

Horsetooth has the biggest potential, as there are lots of areas there all along the lake (well, the dry lake bottom, anyway). there are some places along there where there could be some serious jumping (and I do mean serious). There's another park behind horsetooth but i can't remember the name of it off the top of my head. Most of these places are probably 15 mins off 25.

Most of the places I ride mtn bikes, there are places where there'd be some decent banging around, I just mentioned a few above.

pb