My standby vacation for when I just need to get out of the city and have a break from life is heading into the wild and playing on my mountain bike. There aren't many places a good mountain bike can't go, you are one with nature and you are blazing through it.
The best part about getting into mountain bike is that it is not too difficult to break into the fun. All you really need is a bike and a rack for your car and then the world is your trail. At the beginning, don't spend too much on an amazing bike. Get a feel for the sport before you have the sweetest bike on the block.