Olivia, when we did the HOHO tour in Savannah last year, I think it only took about 1 1/2 hours to get through all 14 stops (depending on how crowded it got). It is a good option for someone who wants to be very independent, but I did see some other tours which look good as well. This will be a bit long, but here goes:

Savannah Tours:
Private Historic Overview: a simple trolley tour with minimal walking and only 1.5 hours long...probably not a great tour if you really want to get the in-depth history
Paula Deen Tour : I don't get this one, as you don't even go to her restaurant (you eat at Bubba's Oyster House instead!)...I wouldn't bother
Savannah Experience : This is the most comprehensive tour, and is in a mini-bus rather than a trolley (which can be a bit uncomfortable). This is the one Mike and I would probably do, if not the HOHO bus
Tybee Island, Beach, and Fort Jackson : It is a long ride to the beach, and quite frankly, there isn't much to see there. Fort Jackson is interesting, and you can still see the cannonballs embedded in some of the walls, from where the Yankees attacked from the river. But this is more a tour for those who are really interested in military history
Mighty 8th Air Force Museum : Another military museum...Mike and I were actually assigned to 8th AF when we were in the Air Force, and I don't care to see it!!! Really for military buffs

You can also do a Savannah Highlights tour from Charleston when you're docked there, but there is so much to do in Charleston, I don't think I'd bother doing that

Charleston Tours:
Historic Charleston City Tour : The shortest of the Charleston tours...probably best for those who really don't care to spend much time in the city
Boone Hall Plantation and Historic Charleston : The historic tour, plus a tour of a "real" plantation...good if you're interested in life back in the plantation days
Historic Walking Tour with Nathanial Russell House : You will see a lot less on this tour than on one of the bus tours, but you will also get more up close and personal with what you do see...it would simply be personal preference if that is what you want
Fort Sumpter and Historic Charleston : Fort Sumpter is where the Civil War began, and this is a must do if you're a Civil War fan. It really is an excellent tour, because it combines Fort Sumpter with the Historic tour...but you've got to be interested in history to enjoy this one, I would think (the boat ride to the fort is really beautiful, though, and you get to see the gorgeous houses along the Battery, from the water). Mike and I would probably do this one, even though we've been to Sumpter before (either that, or the Plantation one)
Historic Charleston Carriage Tour : I wouldn't do this one as a stand alone tour, but it might be nice if you can do it in conjunction with another, more comprehensive tour
Ghosts of Charleston Walking Tour : This is an evening tour, and sounds like fun, but again, only if you can add it on in addition to one of the more comprehensive tours
Aquarium and Sea Turtle Hospital : Unless you're really an animal lover, I wouldn't do this one, unless you've been to Charleston before and have seen everything else there is to see
Magnolia Gardens : Same as aquarium...I wouldn't do unless you've been to Charleston before

Hope this helps. Weather should be great there whether you go in October or April (although it will be getting hot in April). Email or message me if you have any questions or need any other information.