I have recently discovered that I really enjoy primitive-looking, voxel-based, sometimes-sidescroller-y survival games. It sounds a bit specific, so I mean games like Starbound, Minecraft, or Terraria. I love the way the graphics look simple (which means easy to run on older PCs) but feature realistic lighting or shading. I especially love the fact that these titles offer a wide variety of open-ended content and keep the barrier of entry low.