Update from FountainB: after further caching, our "best" prize is probably still the "cash." We find a lot of useless items unfortunately. We have started leaving a signature item. I paint small wooden things like hearts or mini signs, so I've taken to leaving them, often with something like "Geocachers are 'Coordinated' written onto them and FountainB SC on them.
My husband's best two are a highlighter and a USA eraser. Not because they are special in themselves, but because they are from two caches that he found the "purist" way--using only the GPS--no clues, logs or printouts. Also, these were in a national forest, and he loves the outdoors. So, I see what you mean about sentimental value.
Thanks for all the responses. They've been interesting. Keep them coming--and let's all try to trade even or up, starting a trend for nicer SWAG.