I haven't taken a ride on the horses there in Napili but, I know the ranch your talking about. I don't know where they take their riders but, I can't imagine it'll be too high. There are foot hills around there but, nothing like Haleakala. I would check with the wranglers there. It also depends on how long the ride is. If it's an all day thing, they might cover a lot of ground in west Maui. If your "rear" is not conditioned for riding horses, I would keep it as short as possible. I ride all the time and if I don't ride for a month, after an hour I get pretty sore, lol.
I would definitely suggest going over to Napili Bay for a day at the beach. I've been going there since I was a teenager. Snorkeling can be great there "most of the time." I've seen a lot of turtles there. It's a beautiful setting with Lanai and Molokai in the background. Beautiful sunsets!!!
I've also done a couple of dives there. Very shallow but, if you know where to go it can be fun! I think I maxed out at 25' there. Lots of turtles there. It's one of Maui's best beaches. In my book it's the best! I have several friends around Maui that have offered their homes to me but, I always find myself going back to Napili......despite the fact that the Kihei boat ramp is about a 50 minute drive at 5AM in the morning, lol. One of Maui's best kept secrets.....at least it used to be. PM me and I'll give you directions on how to get in there and where the turtles hang out.
If you decide to take a drive up into the west Maui mtns. the road gets VERY narrow there depending on how far you go. Barely enough room for a van in some spots. Be very careful up there and take the drive slowly. You will have NO phone service. There's a gift shop at the top of the mountain (can't remember the name and I don't know what the elevation is there) but, you can't miss it. Some local artists have their things for sale up there. Very cool stuff!
I wish I could remember which mile marker to tell you where the Nakelele blow hole is but, it's on the road to west Maui mtns. Pretty cool lava formations there. That whole side of the island is gorgeous!