There are a bunch of useless, but fun items you can obtain within the game.
Chaco's Lipstick: When you first enter Chaco's house, examine her fireplace and then go over to her bed. Select the option to sleep in it and you will wake up some time later with Chaco sleeping next to you and with her lipstick in your inventory.
Curly's Underwear: After fighting Curly and hearing her out, go into her bedroom. I recommend trading the Polar Star for Curly's gun and level it up to 3 in order to get back after obtaining the item. If you open the map, then you'll notice there is a secret passage way, so jump for it and make your way through the passage. Once you are all the way down, move to the left corner and inspect the area to find the panties. Congratulations, you pervert.
Alien Medal: This one is a bit tougher to get, since there is only one opportunity to do so and it's easily wasted. After making it through the bulk of the reservoir, you'll come upon the shark-like boss "Hammerhead". The goal is to beat him without taking any damage, which can be difficult since he has a lot of stray projectiles all over the place. If you do pull it off, then you'll awaken with this extra trinket in your inventory.