If you're looking for a game that you can 100% with no effort, Harm's Way is as good as it gets. It took me half an hour to get all 12 achievements, and I was intentionally taking my time, as the gameplay turned out to be better than I had expected. I don't think I would have minded if I was required to put in another hour or two into this game.
A couple of the achievements you might need to use split screen to get, especially if you can't find any online action (I couldn't). Two achievements are acquired though online multiplayer, but neither require you to actually play a match, just enter the match lobby. So simple.
The gameplay is very straightforward. Being the shooter I found to be more fun and far easier than being a driver. Driving the vehicles was a pain, as all I seemed to be able to do was go really fast until I crashed myself. However, that crash is an achievement in and of itself, and none of the achievements actually require you to finish a race as a driver.
All in all, if you've got 15-30 minutes to spare for an easy 100%, you can't get much better than this. You might even have fun.