I pretty much use all the same treats as everyone has listed. For beef liver and chicken livers, hearts, and gizzards, I'll give them a quick boil (it stinks!) then freeze and slice em up.
I've also used sardines in the past, but those are so gross and smelly it has to be an extra special reward for sure.
peanut butter cookies are quick and easy to bake too. there are tons of recipes you can google.