I'm totally going through the same thing right now. I also try snacks (which I usually save if we have to wait in a line to pay for something). Small toys? (Although my LO usually just starts dropping them.
. Does your stroller have the ability to change the seats in different directions, because we also tried that which I think is helping us. Also, a slightly random thing, but my DH adjusted the tightness of the straps because now that he's usually wearing thicker pants/warmer layers, I think they were a bit too tight and he was just uncomfortable.