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.
(another idea from @wikkit) Best before 1825. Could be sold in small sheets to jewelers.
If you're going back more than a few hundred years, spices & sugar were valued higher than gold. I'd probably take back something more exotic like liquid sucralose, since it's much higher concentration in a smaller package (1 drop = 1 tsp regular sugar).
Obvious reasons. Only useful before 1944, when it became mass produced. Prior to the late 1800's, it would be akin to a miracle and could have impossibly high value in the right situations.
A hand gun of any type, taken back far enough (say, 1000 years), will give you godlike powers. Even with a single magazine, it would have an amazingly high value and is fairly compact.