Recs.
Updated
Spelunky is an open-source procedurally generated 2D roguelike platformer. You platform your way through endless caves, all while trying to collect treasure and destroy enemies. The game itself is very challenging, creating a fun scenario where progressing past your last death is an achievement in and of itself.
SpecsUpdate
Pros
Pro Lends itself well to quick play sessions
At its heart, Spelunky is a roguelike. This means each game session tends to be short, as the player will die quite often. As a result, Spelunky can be played in quick bursts throughout the day. The convenient save system facilitates this play style by making it easy to pick up where you left off.
Pro Progression is skill based, there are no unlockables, you will genuinely need to get better at the gameplay in order to progress
Spelunky gives the player every tool and ability needed to succeed from the start. The only thing you gain from playing is a greater level of skill and experience – both of which will be necessary to progress further than you last did.