To some extent it depends where you will be staying and what time of year you are visiting. You are right to not consider Skye if you are travelling for one day to and from the Central Belt - you will spend most of the time in the car rather than out with a camera. Having only one day doesn't really enable you to see the best of the Highlands, but, depending on where you start it should be possible to find somewhere.
The weather in Scotland can be very unreliable, so I would suggest having at least Plans A, B and C if you want to increase your chances of having weather conducive to anything other than intimate details of streams. I have not been terribly impressed by Loch Lomond for photography so far. There is the lone tree at Milarrochy Bay, but there tend to be lots of people around the Loch so I prefer to head further north/west. Glencoe can be fantastic in the right weather, but in the summer I find the midges unbearable and there have been times when it has been so closed in that all I have seen is the road. The same can be true of Rannoch Moor. On a few occasions I have done the trip through Glencoe and Fort William and along the Road to the Isles (towards Mallaig) for a bit of variety - I do it in a day, but it needs lots of hours of daylight. The Cairngorm mountains tend to be more rounded than those on the West Coast and I find them more challenging to photograph. If the weather is horrible in the West there are places like the Linn of Dee and Glen Doll where you can access the mountains.