A great sandbox game for all ages
RCT2 is a game in which you try your hand at running a theme park, and it includes pre-made scenarios, some of which were designed after real amusement parks (such as Six Flags). It seems simple, but apart from just building rides, you have to also manage your facilities (such as bathrooms), employee wages, and advertising campaigns in order to keep your park running as successfully as possible. The game also has a scenario designer, as well as a roller coaster builder (which is super addictive because possibilities are, literally, infinite). You can share these custom designs with your friends.
It's really difficult to describe the exact type of experience you'll have with this game, so I encourage you to just try it yourself. It's probably really cheap now, seeing as it's 8 years old, but I promise you that if you're into sandbox-type "design it yourself" games, you won't be disappointed. It's better than the sequel, in my opinion, and truly has something to offer for everyone.