Inverness Botanic Gardens are a useful change of pace from castles, battlefields, and Loch Ness day trips. The site is compact, with outdoor planting, a tropical house, and a cactus house that feel especially welcome on a grey Highland day.
It is not a major garden by British standards, but it is easy, calm, and close to the riverside walking route. Combine it with Bught Park or the Ness Islands if you want a gentle half-day without leaving Inverness.