For those that do not have a basement turning the garage into a mini rec-room can be a great idea. Although it will take some planning and possible electrical work the results are worth it. Tile flooring is a great start as it will add beauty as well as durability to the garage. Wall to wall cabinets will help with providing a little storage and if they are custom made room for a tv and entertainment unit can be added. Just remember when turning your garage into a rec-room that there are neighbors near by and sound will travel. It may be a good idea to look into sound proofing the room for added comfort.
Another alternative is a crafting room. Many more stay at home moms and women in general are working from home either doing crafting, sewing or other general work. This could be a great alternative use of the garage when the bedrooms upstairs are all filled up. Depending on where you live and in what climate a heating system might be a good idea to consider if you are going to design this for an office/work type space.
For those that live with musicians this is a great idea. Whether it be Garage Tile Guys musical instruments make noise. If you have no basement and your ears need a break the garage may be a great alternative. As with the rec-room looking into sound proofing the room is a good place to start as the neighbors may also not enjoy the music.
All guys will tell you that they would love a room to themselves. If you have a fellow at home that has been clamoring for some space turning the garage into a guy's pad might be the solution. Installing a bar fridge, lounge, TV and such might just give him and the guys the ideal place to watch Sunday football leaving the house free.