There's a very pleasant-smelling solvent called "Goo-Gone" that removes
chewing gum, asphalt, tar, masking tape, lipstick, shoe polish, and
bumper stickers.  You'd need to test it for safety on plastic, but I've
used it on various surfaces without problem.  It smells like orange peel
(and in fact may be made of that) and I like to use it just for its
aroma.

Matthew Caulfield