Hi Mel
What area of Loch Lomond are you coming to ?
Depending on what particular area you are staying and whether you will have your own transport, you can easily get to Stirling, visit the castle there (William Wallace - Braveheart country) plus good shopping.
http://www.visitscottishheartlands.com/
Glasgow is also easily reached with many Galleries / Museums and a large choice of shopping and eating.
http://www.seeglasgow.com/
Slightly further away you could go to Edinburgh.
Another excellent day would be to go 'Doon the water' (cruise on the Clyde) on the Paddle steamer Waverley, the last sea going paddle steamer in the world, this is one of my favourites, the scenery is amazing especially if you can spend the day on board and go to Tighnabruaich.
http://www.waverleyexcursions.co.uk
Approx 45 mins - 1 hour away is Strathclyde park (near Hamilton) which has a theme park and also an indoor tropical rainforest.
http://www.ama-zone-ia.com/
http://www.scotlandsthemepark.com/
If you want to be more active while you are here
THIS may be of interest to you.
Please just ask if you would like any more specific information.
Have a great time when you are here
Graham