If you are a boy, you must have lots of collections.
If you see something on the ground, or in the garbage that looks interesting, pick it up. This will be the start of a collection. A sticky Popsicle stick? Great! A beer bottle cap? Super! The feather from a bird that was recently consumed by a cat? Make sure you show it off at dinner tonight!
Rocks are great things to collect because they are everywhere. Especially if your neighbor just ordered a load of river rock to use in their landscaping project. No need to inspect the rocks for unique features or particular beauty; just pick them up and stuff them in your pockets. Your mom will find them when she does the laundry.
If you happen to live in an area where air soft pellet guns are used, you are in luck. When the shooters have gone home, comb the area for a treasure trove of little, plastic, colored balls. Carry the pellets home in your hands or pockets and put them in your sock drawer for safe keeping. Or, just leave them in your pockets and let your mom find them when she does the laundry.
Other things you might consider for collecting: marbles, foreign coins (but not domestic coins because you will want to use those for buying candy), bouncy balls, Pez dispensers, Pokemon cards, paper airplanes, pop tops, tin cans, aluminum cans, plastic water bottles, broken glass, sticks, dried leaves, Bakugan, tin foil, sports trophies, sea shells, beach sand, driftwood, Silly Bandz, rubber bands, Webkinz and comic books.
You can organize your collection in one of your mother's canning jars, or a washed out mayonnaise jar. A better option though: just keep your collections in your pockets and let your mom find them when she does the laundry.