I would suggest a garter, corn, or black rat snake. Corn snakes come in a huge variety of morphs and are quite personable. Black rat snakes also come in several morphs but get larger than corns and in my limited experience are more active. Garter snakes come in many colors and are active and diurnal. They are definitely on the small size but make up for it with big character. The species I would recommend are T. radix, T. marcianus,, T. cyrtopsis, or one of the T. sirtalis subspecies. They do seem to do better in groups.