There are many trails, but not many of great difficulty. The best trails close to Great Falls are in the Little Belt Mountains. Yogo Peak and the higher trails are only accesible in late July to September because snow will keep them closed all other times.
The best way into the Little Belts is through Sluice Boxes state park. Then just take off and explore...Majestic Peak, many mines, Lick creek cavern, many things to been seen and explored.
The Great Falls 4x4 club (Cascaed 4wheelers) is mostly into organized racing and does little in trail rides. The nearest club that wheels on a regular basis is the Helena club. They have more trails to choose from as well.