If you don't want to rent a car or worry about trains it is extremely easy to take tours from these guys:
http://www.highlandexperience.com/
I'm not big on tours, I like to explore on my own, but if your time is limited and you don't want to worry about logistics these guys are great. As you see on the website, they have 1, 2, 3 and 5 day tours to choose from. I did the day trip to Loch Ness in February and it was good. They are located right on the Royal Mile in Edinburgh so easy to walk up and make a booking and to find the pick up spot.
Other advice. Do a ghost tour in Edinburgh. There are lots to choose from, but I would recommend the City of the Dead tour. I've done it twice. They take you into a graveyard in the dark and lock you in a crypt. It's fun.
Go to Speyside and check out the whisky trail, if you like Scotch. Beautiful and secluded countryside and friendly people.