Duke Nukem 64

Shoot a fire hydrant or a toilet and press B next to it repeatedly to gain back your health.

Start a game in single player, co-op, or deathmatch mode. Enable all codes except for invincibility. Intentionally get crushed by a door, then quickly press Start and enable invincibility.

Start a game in co-op or deathmatch mode. Enable the "All Items" code, as well as the "Friendly Fire" option. Have one player shoot the other player with the shrink weapon. Enable the "Invincibility" code, then disable "Invincibility" to get a small version of Duke. Note: Weapons can not be used in this state.

Enable the "All items" code while flying with the jet pack. Enable the "Invincibility" code and the jet pack fuel will stay unlimited.