Savu python. They get long but not thick, and are very active. Think of a corn snake in a python body. They have tons of attitude and while they're not as nippy as other liasis they do take a swipe at me now and then, but it's mostly bluff.
A dwarf locality BCI is another option. Same care as their larger cousins but males stay around 4.5 feet and females at 5.5 feet.