A bike or trike is an excellent way to get about in search of geocaches. What are they, you ask? You can find a lot about it on the internet, the main international website being the geocaching.com one. Basically someone hides a box or container somewhere (under a log in the forest, in a crack in a wall, behind a road sign or similar, just about anywhere where the general public can't see it), checks the coordinates for it, then publishes the details and coordinates on the internet. Then you can look up that cache's info, and with or without a GPS receiver you can go and search for it. They can be as small (and even smaller than) a film canister, or as large as a bucket or ammo box, and are often very craftily camouflaged. They contain at least a log book for you to record your find, and often contain a pencil so you don't have to bring your own, and swappable small items, nowadays mostly for kids (at least around these parts). You can also log your find on the cache page on the internet.
These caches are just about everywhere, from Alaska to Zanzibar (seems there are none in Madagascar, last time I looked!). The good thing about them is they get you to interesting places you may have never seen otherwise, and you're getting fresh air and exercise! Some caches also challenge you mentally or physically, so there's an increased sense of achievement when you find them! It can also be frustrating when you don't find them too, especially if they have a low Difficulty or Terrain rating.
I actually started geocaching last year, after mid-September, and without a GPS. I managed to find a little over 200 before I finally bought myself a GPS this spring.