A sustainable way to get around Loch Lomond and the National Park is to take the 4Bs. This is a network of transport options of Boats, Boots, Bike and Bus. The service is offered on weekends and bank holidays from Easter to September and daily during the holiday season July 4th. to August 18th.
The buses have bike trailers and your bike is transported free. Bike hire is available from 'Can you experience Loch Lomond' Telephone 01389 756 251 and from the 'Coach House Coffee Shop' in Luss Telephone 01436 860 341. You can pick up a leaflet for times and departure points from the tourist office or the Loch Visitor Centres. Alternatively you can check out the web site, a particularly good one which includes maps.
If you want to cycle along Loch Lomond, you will be mostly off the road, worry free of the car traffic that can be a bit heavy at times. You will have trails, the sidewalk at times and some quieter roads designated for local traffic. You will be given the best panoramas, as you will be at times right by the water, and some other time, right in a rain forest. If you don't have a bike, you can rent one, so no excuse!
I didn't do this, but you can take a cruise on the Loch on the Maid of the Loch which leaves from the outskirts of Balloch. There are other cruises from the marina near the centre of Balloch and also from other points around the Loch such as Tarbet and Inveruglas