For the mouse, plug in a mouse, and you'll have a better time than using a touchpad.
I still need to try a Chromebook with a numpad... perfect for roguelikes.
I have tested games EXTENSIVELY in the Steam for ChromeOS beta. It's not a powerhouse for modern gaming, but there are many games that run well from up to the 7th console gen. Of course Source engine games run well, but Unreal engine 3 games run solidly too. The hardest that I have pushed it is Dark Souls 3, which doesn't run well, even at the lowest settings (15-40fps), but is stable, which impressed me. 720p 60 is possible across a large variety of games, but 900p 45 is what I am usually shooting for. Bayonetta, Bulletstorm:FCE, DMC, all of the Nightdive remasters, Vanquish, Arkham Asylum, Prodeus, both Portals, Enslaved, and Prototype 2 have all run very well.