Kathy:
it sounds like you're looking for confirmation that a car rental is the way to go. If your mind is made up, then go for it. A car will get you were buses and trains can't--for sure. If by out of the way, you mean the Isle of Skye or more broadly the Inner and Outer Hebrides the Orkneys or maybe even Shetland, think through the arrangements to get a car from wherever you rent it to where the out of the way places are you plan to go. Also, look into whether or not reservations are required to ferry a vehicle, the timetables for the ferries, and the cost of transporting a car.
One another note, I've posted elsewhere on RS Scotland, when we were on the Isle of Skye this past May 5-8, (we didn't have a car) the typical venues (Old Man of Stor, Kilt Rock, Dunvegan Castle, Lealt Falls, Fariy Pools, Quiraing, Fairy Glen, etc.) the car parks (parking lots) were packed, and this was early May, before the larger tour buses and more tourists arrive. By the way, while we were there the weather was bordering on miserable (almost gale force winds and rain most daylight hours) but the car parks were still full. Our driver told us the parking lots will be virtual grid-lock in the high season. We toured Skye in a small van driven by a local. To us (as observers) it was "funny" to watch non-UK drivers (drivers unaccustomed to the steering wheel on the right and the transmission "stick" on the left),, struggle to back-up their manual transmission cars on a hill; they would usually stall the engine 3 or 4 times during the process. Regarding serendipitous stops that a rental car will permit you to do, we're not aware of the roadway layout on the other islands, but for the more out of the way places on the Isle of Skye you can only rarely pull off the road and stop as the roadway pavement is quite narrow and there is no "shoulder" as we're accustomed to in the US.
Regardless of what transportation you use, if you're in the West Highlands in June be aware you may encounter midges.
Have a great time.