Inverness is the capital of the Highlands in the practical sense: trains, a river, a castle on a hill, and roads that leave for the west and the far north. It is not pretty in the Edinburgh way. It is the correct place to sleep if tomorrow you are going to Skye, Ullapool, or the Cairngorms.
Give it a day and a night. The mistake is to treat Loch Ness as the attraction and the town as a garage. The loch is a long body of water. The town is a riverside walk and a good supper.
See
The river and the islands
The Ness Islands are a short walk from the centre and the best half-hour in town.
The castle viewpoint
The building itself has been a court and a workplace. The view down the river is the point.
Culloden, if the history is yours
A bus ride east. The battlefield is well explained and not a theme park. Allow the morning.
A boat only if you want a boat
Loch Ness cruises leave from Dochgarroch and Drumnadrochit, not from a monster ticket office in the high street. Go for the glen, not for a sighting.