Something which is pleasurable actively GIVES you pleasure – it has to do
with your senses.
Something which is pleasant is something which is nice, but it doesn't
necessarily give you pleasure.
„Pleasant“ is a very neutral adjective. It means that something is nice
or perfectly acceptable, but it's nothing extraordinary.
„That was a pleasant way to spend the afternoon“ is a nice way of saying
that the afternoon was nice, but you actually might have been a bit bored. It
wasn't horrible, but it wasn't something you're going to remember forever.
„That was a pleasurable way to spend the afternoon“ means that you were
doing something that actively gave you pleasure – it was enjoyable, probably
fun, it made you feel good.