Mine Falls park is one of the very best things about Nashua. I live very close to it, and walk, run, mountain bike and take my dogs out in it. it is great for all those things, though for mountain bikers looking for highly technical areas, this is not the place. It is very large so there's plenty of room to roam, and home to a variety of flora and fauna, though it does get crowded by running groups late afternoon and early evening.
this is truly one of the best parts of Nashua. Just be sure to 1. carry out what you carry in, and 2) stay to the right, lest you be run over by bikers.