Alec has already told you to play XCOM this weekend if you haven't, seeing as the full game's free to try on Steam. It's "one of the best games of the last few years" says he. Aye, maybe it is, and maybe you could. However. If you fancy real-time smashing rather than turn-based tacticisicing, have a bash at the superpowered open-world antics, shenanigans, and - dare I say - bants of Saints Row IV [official site], which is also holding a free trial weekend on Steam, as is Saints Row: The Third.
SR4 is the best action game on PC, according to John.
Saints Row 2 is still my favourite in the series, but IV comes a close second. With only two days to play for free, I'd skip Saints Row: The Third and go for IV, as it's basically The Third with added superpowers and a full embracing of the campness and silliness that The Third didn't go quite far enough with, which left it feeling a bit tonally weird. IV's superpowers are so much fun, giving abilities like telekinesis, super speed, super jumping, gliding, superpunching... it's still bizarre to me that the best superhero game anyone's made in years and years began life as a fairly straight-faced GTA clone.
The Third and IV should now be in your Steam library, waiting for you to install them, or they're here and here. You'll have until 9pm UK time on Sunday to play as much as you please, and all things Saints Row are heavily discounted in a weekend sale until Monday if you want to keep them. Or, if you already have 'em, all the DLC's on sale too.