Depends what you want. Culture, shopping and cafe society, go to Edinburgh. Scenery and the great outdoors, the Cairngorms, the Northwest coast, and the inner isles. If you're a whisky fan, tour Speyside and do the odd distillery tour. Inverness and Loch Ness also worth a visit.
If you're lucky with the weather and in rude health and fitness, a day spent scrambling up Ben Nevis rewards the effort with magnificent views. A less strenuous option is to take the Cairngorm mountain railway. Pleasant cafe/restaurant at the top.
A wee word of warning which, not surprisingly the Scottish tourist board don't mention, avoid the mountains and the west coast from June to September, as you're likely to be eaten alive by midges. If you must go then, pray for a breezy day.