One of the easiest conversions you can do is to convert your garage into a workshop. You don't necessarily need to finish a garage by installing walls and a floor if you're going to use it for a workshop, as you would for other purposes. Build and install workbenches to provide you with a surface to work on. If you have large tools like a table saw, you can take advantage of the open space in the garage to use them. Hanging a pegboard on the wall provides you with a place to hang your smaller tools. And you can still bring your car into the garage to work on it.
Some people convert their garages into a home gym. The open space and fairly high ceilings found in garages provide room for exercise equipment, as well as room for you to move around without fear of hitting a low ceiling as you might in a basement gym. Most exercise equipment doesn't need to be plugged in, so you most likely won't have to install new electrical outlets. Insulating the walls and covering them with drywall can make the gym more comfortable and inviting. For the floor, rubber interlocking gym flooring can help protect you while you work out, and be more comfortable than the bare concrete floor.
If you own a small business, or have clients meet you at home, converting your garage to a home office can be an option. Your garage may already have an exterior door for people to enter and exit, which helps separate your office space from your home. Having that barrier provides you with a level of professionalism you won't be able to get meeting with clients in your family room. Finishing the interior is a must, and you should either remove the door for the car, or cover it up with an interior wall, and install a carpet or hardwood floor.
Another option for a garage conversion is turning it into a spare bedroom. This is useful if you need the extra bedroom for your kids, if your parents or in-laws move in or as a place for relatives or out-of-town friends to stay when they visit. Again, the interior needs to be finished with insulation, drywall and flooring, and you need to remove or cover up the garage door. You may also want to install a bathroom if someone is residing in the bedroom permanently.