I have been a bird watcher since the late 1980s and always carry some version of optics with me. I have had three pairs of binoculars and four monoculars. I currently own one pair of binoculars and two monoculars.
My monoculars are 8x25, and my binoculars are 10x42. I use the 10 power primarily for distance bird watching, for example along the coast. Any magnification greater than seven can make it difficult to find birds in foliage. Eight is somewhat of a compromise.
I also carry my monocular for scouting cross country and climbing routes. I have not found it necessary to have a 10 power for this. It's completely unnecessary to have a zoom on a pair of binoculars - it makes them heavier and less effective.
The reason I carry a monocular hiking is that it weighs less than half of a pair of compact binoculars. The reason I have two is that whichever monocular you carry with you hiking is going to get salted up. The other one I keep in my truck very handy. It will become my primary hiking one once the hiking one gets salted up.
If you choose to get monoculars for a hiker, I would recommend 8x35 or 8x25. 7x is also a good option, particularly for a birdwatcher.
Choose a somewhat reputable brand instead of paying extra for a zoom that is unnecessary. This is a great gift idea for a lot of hikers, so you're on target.