Going to include a list of the things I have used at the end of this post just to show how much variety there can be
I give the tiniest pieces as far as treats go so the boys don't get full too quickly. I also have adopted a tailored version of the "nothing in life is free" school of thought. The boys get loves and play time whenever, but if a food reward is involved, they are always doing something to earn it, even if that is just holding a sit-stay nicely or doing an easy trick. LIMA-type trainers are often painted as "cookie pushers", and I would really like to avoid that (though Nino, who is skin and bones, could probably use some cookie pushing
Wellness WellBites, Sojos Simply, Bixbi Pocket Trainers, CloudStar Tricky Trainers (not the cheddar...not soft enough to make teeny), Northwest Naturals treats, Nature's Variety freeze dried, Stella and Chewy's freeze dried and frozen morsels, Happy Howie's roll, liver bait bought in bulk from show vendors and frozen, BilJac Lil Jacs, all-beef franks baked within an inch of their life and frozen for keep, rotisserie chicken (both baked and fresh), various deli meats (again, baked and fresh), Vital Essentials rabbit bites, Zuke's Hip Action, various Plato treats broken up...
And those are just my most consistently used ones. Inside all of that is a hierarchy from mid-value to high-value. All are soft or freeze dried as neither will touch biscuit type treats. Another thing that has been amazingly beneficial, especially for agility training, is creating a tug drive. Tug is one of Nino's favorite rewards and it has made creating speed and focus in weaves very easy.