The first time I drive to Rosapenna it was an evening, the sun was setting and I was driving through the spectacular Glenveagh National Park when I suddenly came around this bend and was hit with this incredible view.
The view point at Lough Salt will blow you aware (literally due to the wind and figuratively). If you can catch it on an evening as the sun sets on the way to Rosapenna it will become the moment you realise you're in a special place.
You can do some hike in this area but simply getting out of the car for a look will be enough for you.