I suspect you'll get a myriad of replies with different opinions.
Of the snakes I've observed (nonvenomous), few are more active than the colubrids. I read a lot of funny stories posted here and elsewhere about King snakes and their "I'm the KING" attitude. I'm not sure how harmless it is because they sometimes decide YOU or your finger/hand are a food source and there is a recent thread on another board where owners talk about the bites they get from their king snakes and how waiting them out to release isn't always a given. They'll wrap and chew and actually keep it up. Harmless yet, distressing I'm sure after a while.
I'd love to have one. Truly they are harmless, but they like to eat and being quite active they eat more frequently than most boas and pythons.
Diurnal snakes are usually pretty active. Indigo snakes are quite entertaining yet rarely bite, and I know a few people who just love them. They are a bit expensive though.
I've chatted with some folks from Australia and asked them what they felt were some of the most active pythons from their land and though there were different answers, the majority felt scrub pythons were fairly active, based on python standards.
I'm not going to call them harmless as they have the same potential to make a mistake, or I should say WE have the potential to make a mistake dealing with them and they can get rather large depending on the subspecies.
Retics seem to be rather busy as well, by boa and python standards, so maybe a nice super dwarf would be something to look at.
Of the 3 species I own, I'd say my coastal carpet is the most active, and also the most seen, then the boa constrictor and then there is the royal which is not very active. None of the 3 mentioned in my collection are going to be like a colubrid.
I'll bet you'll get some good info here once a few others chime in.