Great for fun although it's not hugely difficult, try
Plants vs. Zombies by Popcap.
Plenty of free online games too. I know several people who play Ogame (
www.ogame.org) which is about building a space empire. There's the old classic Urban Dead where you get to be a zombie or survivor (
www.urbandead.com), there's a more detailed and rather superior game along similar lines called Nexus War (
www.nexuswar.com) although membership is in precipitous decline. There are always also fun little online games like Desktop Tower Defenc(/s)e, or Spin The Black Circle, all easily Googleable.