I've done this but not on a cruise trip. At the tectonic plates we were given the option of staying on the bus or doing the walk. It wasn't a very long walk but if I remember correctly there was a bit uphill at the end. At gulfoss waterfalls you can walk down or not to nearer the water fall but you can see quite well from near where the bus is. There is a little bit of walking at the geysers but only a very small distance. It's worth finding out what the tour operator advises because its really is one you'd not want to miss. Well worth seeing.