Recommend the Loch Ness Centre in Fort Augustus rather than Nessieland, if you are a Nessie buff. There are beautiful hiking trails and drives all about Loch Ness, and a boat trip on the loch is a must.
Call ahead to make sure they are running all the tours, since most companies only take the boat out if they have a minimum number of passengers. It is also pleasant to just sit by the river and watch the canal traffic or wander through the shops. Urquhart Castle near Drumnadrochit is well worth a visit for its views alone. Tickets include a short video on the history and the ruins are well-maintained and labelled.