Chocolate + sugar + peanut butter. A winning combination throughout the ages. Even people who can't understand your language can immediately determine the value of a peanut butter cup. It's Prime Directive friendly (difficult to reproduce and unlikely to affect the course of other recipes), consumable (fewer twonky problems), cheap enough (for the traveler) to give to a child for a quick bit of information, and valuable enough (to the local) to barter for items of immense value.
(idea from @wikkit on Twitter) A 350-Farad capacitor is small enough to fit in the hand and available for $11. An electrical engineer from the mid-1800s to mid 20th C would easily be able to verify it and would value it highly.