I'm a little hazy on the timing to get to the volcano, but we stopped at the butterfly farm on the way up and walked up to the volcano and back down just as the buses got there, so I think it was somewhere around 1-2 hrs. Then on the way back they took us to few places including a "metal" church and then Saarchi for lunch. We bought the driver and guide's lunch and enjoyed their company. Then back to the ship, so probably gone about 6 hours.... I think for the seven of us it was about $35-40 dollars a person, but much less than the tour bus (it is negotiated at the pier... early off gives you pick of the litter so to speak.) We did give the guide a tip at the end and something for the driver to ... it was well worth it. With the stops it wasn't that bad and when you get into the mountains the weather is cool and perhaps a little rainey... in the 70's most of the time. Hot and humid at the port and just remember that the rest of CR is much more lovely than the port is. (Boy is that an understatement) They do have an interesting flee market right next to the pier, but very touristy trinket type stuff.
By the way Cartenega blew me away. That was the surprise of the whole trip... from the sea it looks like the whole city is just sitting on the water.... a wonderful place.... to bad about all the problems that are going on, though we never felt it when we went there.