I can understand you not wanting to use the harness. My son can be a pain when we have him in his! When we have him in his chair we normally try to have a supply of food and drink that he likes, a book or magazine might be nice for him to look at, sometimes a new (inexpensive) toy can work wonders. Also you could try playing an 'eye spy' type game with him and get him interested in what he can see.
The weirdest things can keep my son quiet though. Last time we went shopping and he was in the trolley he wanted to hold a box of cereal and there wasn't a peep out of him after we gave it to him!