If you want a fun FPS, check out Sanctum 2. It mixes FPS with tower defense, sure, but you are usually severely resource-constrained in terms of the towers you can build and have to mop up quite a few baddies that make it through your gauntlet. It requires some high-level strategic thinking in terms of weapon, perk, and tower load-out, how and where to spend your resources to place towers, and tactics against the enemies that make it through. But it's pretty much jump in and have fun.