I'm fairly young, but I'd suggest Lego due to its versatility and cheapness. However, it would be best to get your Lego from non-official retailers such as Bricklink or Ebay if you want to build your own stuff. Some of the other hobbies mentioned here are doable in Lego, such as model railroading and miniature landscaping. I've seen some cool mechanical stuff made with Lego too, such as various robots using the NXT system, RC vehicles and motorized displays, ball machines, and other non-motorized contraptions. However, like the other hobbies, it has the potential to cost you a pretty penny, and it can take serious skill to make a good build. The community of adult Lego fans is pretty decent from what I've seen, and sites like Eurobricks or The Brothers Brick are examples of the virtual presence of the Lego community. It should also be noted that it may take extreme patience on the part of yourself and your family to deal with Lego due to the potential for a vast bulk of bricks taking up space, sorting and organizing bricks, etc.
