You can do rafting and zip lining out of either La Fortuna and Manuel Antonio and both offer great experiences so you can’t really go wrong. But for zip lining, some of the tours in La Fortuna have a view of Arenal Volcano, if that makes you lean in that direction. For rafting, the Savegre River near Manuel Antonio is fun, but depending on weather, the water level could be on the low side at that time of year so maybe do that near La Fortuna. There are a few different options for places to raft around Fortuna so that’s a safer bet if it happens to be very dry when you visit. Hope that helps. Have a great trip!