Maybe spotted / children's pythons? I've heard good things about them and they stay small. Sand boas stay small, but you won't see much of them.. They are fine to handle, but stay buried a lot, so not good "watching" snakes. There are some awesome KSB morphs though.

For a breeding project, cresties could work, but there are so many people breeding them now. Unless you just want to produce and keep the offspring, I wouldn't breed them intending to move the offspring. There are also some very flighty cresties out there. I have a nut myself that flings himself into the air, solid objects, anything when I try to handle him.

Have you considered chahouas / "chewies"? They have great patternings and stay a little smaller than leachies. They supposedly have more personality than cresties and they hold good market value.

Personally, I agree with the smaller boa idea... Would he be OK with something that stays BP sided or smaller?