
Yet another suggestion, but this one is non-toxic (although I would handle
it like any other cleaner) and it smells nice.  The chemical is called
d-Limonene, and it smells like lemon peels (I think it is a lemon extract
of some kind).  You might find it in a hardware store, sold as a
general-purpose solvent, but your best bet would be any bike store --
it is used as a chain-cleaning fluid.  It works on sticker glue, crayons,
grease, oil, etc.

It's clear yellow in the bottle, and you can use it straight; if you add
water, it changes to a creamy consistency, which is good for some jobs.