I'd say to download NES/Genesis emulators, they have some good games.

welp, AssaultCube used to be my favorite game when i had my poorest pc ever.
PowderToy if you like wasting time(it's mostly a simulator, but a pretty cool one) with chemical reactions.
Minecraft is the best option i can think, it's really fun making stuff and then exploding it. Or just adventuring with your wolf/dog
Half-Life 1 is also cool