I like to use natural balance rolls cut up into treats. They have a soft texture and are smelly and flavorful enough so Tucker likes them but not so messy that I can't keep some in my pocket. They are great, can be any size you want, easily broken into smaller treats. Only down side is they will dry out if out of a bag too long.
So for my bowl of treats in the living room I use freeze dried treats (meat, liver), dehydrated/freeze dried raw (stella and chewy's, nutrisca), sample kibbles (so not what he normally gets), zukes treats (though those dry out too and their value falls to that of kibble), and jerky treats I make and break into pieces (you just thinly slice meat and bake on a lower heat for three hours or so). The jerky is great for working on not barking out the window because it takes him a while to chew it, not so good for training sessions though.
When I go to the park I usually just bring leftover meat from the dinner before and if I don't have that I'll use "junk food"...cold cuts or hot dogs (which get pretty gross in a baggy in my pocket for a couple of hours lol). Right now I just use his meals for training commands because he gets so many treats throughout the day, but before I would also use leftover meat, cold cuts, or hot dogs.
String cheese is used for nail clipping, he loves that stuff.