The natural soundproofing that is inherent in most basements makes them an ideal place to position your dream of a home theater. Add some carpeting and upholstered furniture to help with the acoustics. Don't hang anything but movie posters on the walls and add a dimmer switch to ambient lighting to cut down on glare. A small kitchenette would be useful for providing snacks and beverages that don't require going upstairs.
The basement can actually become a large kitchen and dining area if your upstairs kitchen feels cramped. The expansive space in most basements allows for cabinet and counter space. The toughest part may be getting appliances like a refrigerator and stove down into the basement, but if you can pull that off, you can then set your sight on redoing the old kitchen into a completely new and different type of living space.
The kids will appreciate having a separate space to head down to for playing video games, table tennis or pool. Many basements have been converted into playrooms and that soundproofing means that the kids can turn up the volume when playing Rock Band or Guitar Hero without disturbing the rest of the house.
If your laundry room consists of a closet-sized area into which the washer and dryer fits, you should consider transforming your basement into your own personal laundromat. In addition to the washer and dryer, you might want to consider installing a fold-down ironing board and stringing up a line that traverses the room for air-drying fabrics that don't deal well with the heat of the dryer. Another good idea is to create a laundry chute upstairs that transports dirty clothing down into a bin inside the basement laundry.
A home office is a very effective design idea for the basement. The basement office provides space for a large desk, filing cabinets, computer hardware and a printer plus paper. If your current home office is really just a laptop and file cabinet inside a small nook of your bedroom, you will truly appreciate the space afforded by a basement home office.
Install your exercise equipment in the basement and put up a television to help you pass the time more quickly. A home gym inside the basement takes advantage of the cooler temperatures provided beneath the surface of the ground when you start working up a sweat.