Check with the local dive shops as to what places will be safe to snorkel during that specific season. The drive to Hana is slow but otherwise easy. You want to have
some time to stop here and there. If you go almost to the end of the paved road to the NP parking lot, forget all that Seven Sacred Pools nonsense and head uphill instead, if you don't mind a decent hike to some much more impressive waterfalls. Driving clear around the island on that dirt road is somewhat slow, and will take you past dry more desert-like conditions. That might make for some awful long drives for a honeymoon, unless you have nothing better to do! The summit of Haleakala is a marvelous spot, especially if you're up there at dawn (if you have nothing better to do). Watching the sunrise over the Pacific is like witnessing the
birth of the world. But remember that the summit is up there, so a good windbreaker and sweater might be mandatory. The tour buses and crowds arrive by midmorning, right around when all the good light is gone anyway. The drive around the north side of the island takes a quite a bit more nerve than the Hana road,
which is a freeway by comparison, but does have some very different sights. We're going back in Dec and staying in Napili again.