Most video games hide a few secrets for players to find, and often those secrets are worth the trouble of discovering them.  Sometimes a game goes above and beyond, hiding the goodies past the point of reasonable actions.  GamesRadar has put together a sampling of games that feature some neat secret stuff, but you'll have to jump through some bizarre hoops to get to them.  Consider how much 2004's Ninja Gaiden for the Microsoft Xbox makes you work to unlock the hidden 2D original trilogy:

Instead of making the classics unlockables for finishing the game (no easy feat in and of itself), you have to undergo some weird trials to get each game one by one. The first game is the highest mountain to climb; you have to snag each and every one of the 50 hidden Golden Scarabs and trade them for the OG NG. For the second and third original titles, you need to shoot an arrow at a clock and revisit a spot where you had found a particular Golden Scarab before. At this point, you can play them at Ryu's safehouse. If you want to access them via the main menu, you have to complete the game after collecting them. Okay Team Ninja, everybody gets it; your game is hard.

I don't think I'd even bother with that.  I was always fond of the hidden versions of Donkey Kong and Jetpac in Donkey Kong 64 for the Nintendo 64, and while the legwork required to get to them was a challenge, it wasn't too unreasonable.  Players had to collect the special Nintendo coin in an in-game recreation of the original Kong arcade game in order to play it on demand, while Jetpac required snagging fifteen banana medals during the main quest.  I spent a lot of time playing both of those retro games instead of the main Kong adventure.  They made a nice break when collecting bananas, medals, coins, blueprints, and everything else the story required became tedious.