Just since they're very unusually kept and I basically never see them recommended anywhere ever: northern pine snakes. Mine is a growing juvie, but has already been the most interesting snake I own. Docile, and alert and comfortable with sitting near me and looking around. Easy husbandry.
My female comes out in the mornings and afternoons to watch the world, and returns to her hide at night. Mine shows very obvious learning capability and clear behaviors, appearing more intelligent than other snake species I've kept.
She becomes obviously comfortable with me with regular handling, and now comes out onto my hand on her own when I open the cage, like a little cat. But when I went on vacation for a couple weeks and came back, she totally forgot me the first day! Hissing and rattling like I was a stranger LOL. Then became my friend again/remembered me and hasn't turned back.It was only for that first and second day that she hasn't seen me, so clearly a recognition/learning behavior.
She also rattles at any other person in the room that she hasn't seen before- but is comfortable with me, whether I'm flailing my hands in front of her or sticking a camera in her face. She is very discerning. But since I've moved her downstairs to a room where she sees people and dogs every day (since she comes to the front of her tank to watch the room every day), she's been comfortable around others as well. It's pretty cool!
Since you want a smaller snake, get a male than a female. Females grow up to around 6-7 feet, but males are much smaller.