There are several customizable arrow modifiers that can be turned on and off and combined to change how the game plays. These modifiers can affect the look of the arrows (and whether you can even see them at all) as well as the locations of the targets and other factors about the way the arrows move that can increase the difficulty of playing and change the overall experience. When you combine these modifiers together, the gameplay can get quite maddening. You'll want to play around with them a bit to see what is fun for you and your crew.
One interesting modifier that can be selected will randomize the steps. This basically means that in a given song at a specific difficulty setting, whether there was a single step, a jump, a hand, a hold or a mine, the same type of move will be used, but the specific directions involved are randomized on a per step basis. Put quite plainly, you can test your skill against other players without worrying about them having memorized the moves. Of course, you'll have to read as you go, too.
The Fitness Mode is a nice feature, allowing you to set a goal and to use the game to work out until you reach that goal. The game even keeps "score", showing you the number of calories you've burned so far. This would have been a much more useful feature, however, if it were possible to switch back and forth between stored profiles or to play a multiplayer game in Fitness Mode, switching back and forth. I find that my wife and I will combine our caloric goals and then just take turns until we meet that goal, but that's not as accurate as it would be if we could switch rapidly back and forth between profiles.
And for those of you who are more fit and have developed some skills at these sort of things, In The Groove supports play with two pads as well - either for competition or single-player gameplay across both dancepads. If you plan to attempt playing across two pads, be extra careful that they stay put; many of the dancepads on the market are thin, flimsy and can slip around if you are exerting force from the side. Personally, I played In The Groove on one of Red Octane's Ignition Pads - that's a fine piece of pad.
One interesting mode available in In The Groove is the Marathon Mode. In Marathon Mode, you select a "course" of similar songs that have been grouped together. These songs are then played back-to-back, allowing for a nearly non-stop gaming experience and avoiding the long load times that occur when your scored is tabulated and you have to go back through the menu system. Be warned - some of these "courses" are grueling. They'll start with some offering of hope that you might make it through and then ramp up in difficulty pretty quickly. The "Marathon" mode feels more like a "Survival" mode.