I agree with the others that an adult boa will be a little crowded in a 55 gallon.
A pine/bull/gopher snake could be OK, although I think many subspecies get a little large for that size cage. It would be long enough, but is a tad narrow. I do like the mexican bullsnakes (P. deppei ssp.) but they are a little bit "temperamental" and probably not a great starting snake.
I think one of the North American Ratsnakes would be a great choice. They are a little arboreal, particularly the yellow and everglades ssp., get big enough to be interesting, are active, docile, easy to feed, cheap and colorful. You could probably keep a pair of adults in a 55 gallon tank.
Baird's ratsnake is another popular choice, but they tend to be more reclusive.
I don't know what makes ball pythons boring? They are beautiful snakes. They are shy, sure, but certainly not boring.
What about a pair of Spotted Pythons? Very hardy, docile, and easy to keep and breed.
If you really want something more arboreal, you might consider a jungle carpet python. They are delicate and occasionally snappy babies, but grow into trustworthy showpieces that coil up on a branch "on display".
There are more choices that we have time to discuss.
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Chris Harrison
San Antonio, Texas