A great way to waste time is with World of Tanks. It is free to play MMO, with options to pay (you pay for gold, and with the gold you can earn XP and credits faster, buy premium tanks, buy premium ammo). It covers tanks from the Second World War and shortly after.

Your performance in each match (of 15 vs 15) determines the amount of XP and credits (money) you earn. You then spend that XP to research upgrades for your current tank, or research a whole new tank. The credits are used to purchase the upgrades or the new tank. As each tank crew gets more experience, their overall effectiveness improves until it hits 100%, then you choose which of several categories each one works on next: repairs, firefighting or camouflage.

Think of something like WoW where you just run battlegrounds all day, or Rift/WAR where you just do scenario after scenario. There are some other battle modes (such as Clan Wars, in which each clan is fighting on a persistent map to gain or defend territory), but the normal is a 15 v 15 capture the flag (or win by killing all the bad guys) battle.