I like to leave things that are particularly usefull to geocachers. I carry around a variety of stuff in the $1-2, $3-5, $5-10 range and for special trades $10-20 range. I always carry around several TB's of my own to trade for others. I also like to keep in mind that many cachers bring their families with them, so I carry around a few kid items. BUT all of the stuff is usually new and of good quality -no junk. And I don't spend a ton, I just pick up things on sale when I find them at Wallmart, the Dollar Store, etc. I like to keep the items small and realitively flat so I can eaisly pack them and take them with me when I travel.
$1-2 Range
Coleman Zipper Pull/Keychain Compass & Thermometer
Pin-on Ball Compass
Small Bison Containers
Small Bison Survival Whistles
Batteries
Carabiners
Mini-lights
Bug Spray
Camo Tape
Magnetic Badge Backs (the good type that can be used to hold up a fairly large container -from K&J Magnetics)
$3-5 Range
Coleman Map Compass
Large Bison Containers
Bison Ball Containers (some of the best micros)
Large Bison Survival Whistles (the end is also a micro container)
Coins (military unit type)
Patches
Keychain Calculators
Sunglass Hut Sunglass Visor Clips
Sunglass Bags (you can put anything small in them like a GPSr and they are great for cleaning the screen)
$5-10
Gift Cards (Lowes, Home Depot, Wallmart, etc.)
Small Maglites
Adventurine & Mother of Pearl Stretch Braclet
Geocaching.com Lanyards
Good brand name ear buds
$10-20 Range
Any cool small tool/electronic
Leatherman Micros
Motorola Talkabout Radios
Pretty decent Sony, JVC or equivalent mini headphones
Very small USB Hub
Winchester Multi-tool (very good quality and for around $15)
Packs of batteries
Tiger Eye Stretch Bracelet
Kid Stuff
Scrunchies (not the generic type, but something with beds and bangles or college logo print)
Funky beaded necklass kits (I especially like the ones that come with a frog bead that I find at the Hallmark stores)
Magnets
Giant Crazee Beans
Small Beanie Babies
Lurchin Urchin Yo-Yo Balls
Super Balls
Logo Rubber Stretch Bracelets