A mini first aid kit is a great bit o' swag to leave. I often rubber band a few antiseptic wound wipes and bandages together to drop in caches. Those pin-on bug repellent badges or a spare set of AA's can also be great swag. If I could find single use packets of sunblock I would stock up on those too.
It's nice to leave/find something useful in a cache otherwise full of broken toys and bent trinkets.