I'm having alot of fun with Arma2 at the moment. I have the full version, I'm not sure if the demo has this, but in the full version you have an "armory" mission type.
But what you do is start with about 10 things to pick (be it a rooster, rifleman, boat, whatever), you pick one, and get into the game. You get challenges based on what you pick, and you have to achieve a goal. Once you do you get "points" and unlock more items. You can just keep going and keep beating challenges and unlock points to unlock new options creatures, guns, airplanes, whatever.
For example. I picked up a PBX, a navy seal boat for landing on beaches. THe first challenge is, stay above 50kph on this boat for 30 seconds. Ok, I do that, get 10 points, then get a challenge, do a 180 within 5 seconds. Ok do that. Then "see how far you can make this boat skid up on land" I failed within the time frame because I was actually driving out to see the carrier in the bay.
Picked up a rifleman. Got a "lets go hunting" mission. You have to run around the mountains killing sheep, cows, etc, and the more you hunt, the more points you get. Then got a "shoot skeet" mission and got to shoot at clay pigeons.
Kind of reminds me of flight simulator type missions.