The main meaning of regal' is indeed to entertain, as in treat someone to some sort of ongoing process of entertainment. This often has the connotation of food being involved, although strictly speaking that's not important as the treat in trick or treat doesn't have to be food related. At least not in the UK anyway.

However I've seen regal' used many times to refer to a treat that isn't delivered in some kind of extended entertainment process, and has the idea of a one-off gift type thing; pretty much the same thing we mean in English with 'treat'. I don't have specific examples to hand, but I've seen things like regalis ŝin per donaco (treated her to a gift) and similar usages. I believe if you search in http://tekstaro.com/ you'll find that kind of usage, alongside its more commonplace meaning of entertainment.