A wide variety of accessories are available for bicycles, some of them quite affordable. Here are some inexpensive accessories to consider adding to your bicycle…
BOTTLE HOLDER: A bicycle bottle holder, or "cage", can be purchased for under $10 with some exceptions (like ultra-light models or units which come with a bottle). Some of these can hold regular twenty-ounce soda bottles; re-using a bottle of this type for water, if desired, is cheaper than buying a new bike water bottle. Usually they are held on by two screws, make sure it is possible to install one of these on your bicycle before purchasing it.
BICYCLE LIGHT: Some flashlights which can be mounted on your bicycle cost under $10, sometimes as inexpensive as $7. They may use LEDs (Light Emitting Diodes) or traditional light bulbs. LEDs use less electricity and usually last longer. Some bicycle lights have a feature which makes them repeatedly flash when desired; this may be useful when there is poor visibility while bicycling.
FRAME PUMP: This is a type of bicycle pump which can be mounted to the bike's frame. Many of them are relatively inexpensive, at about ten to twenty dollars. These accessories are convenient, as they allow the tires to be inflated during travel, instead of having to return home. If a tire is punctured and starts slowly leaking, this can enable you to reach your destination without having to push the bicycle on foot.
CAMERA: Bringing a camera along while bicycling allows you to photograph scenery, animals, vehicles, or other subjects of interest. An inexpensive compact 35mm camera which doesn't require batteries won't add much weight; most of the time when you are bicycling, you aren't likely to need a flash. These can be purchased at yard sales, eBay.com, and thrift stores for as little as $5. A waterproof camera (somewhat more expensive) or sealed container will prevent damage if it rains.
BICYCLE LOCK: Although some of the more sophisticated units can be costly, many inexpensive (under $15) bicycle locks are available. Locks can be purchased which are unlocked using a number combination, while others use a key. Many key-based bicycle locks come with an extra spare key.
MIRROR: Bicycle-mount mirrors can be purchased to increase rear visibility. Some, like the VuBar, can fit inside the end of your handlebar, while others are mounted on the handlebar. Inexpensive units are available for under $15.
All of these inexpensive accessories can be purchased on eBay.com, as well as local bike shops and some online shopping websites.