You can fill space with plastics plants and vines. And maybe get him something to climb, they're very good climbers and it's good exercise for them
Far as larger snakes go, any snake can bite without warning but boas are generally ok and would be a good snake to go for. Carpet pythons would be a good one as well
Rat snakes get bigger than corns, beauty snakes can reach 10 ft+ but stay slimmer.