Loch Lomond is the second largest loch of Scotland, the largest body of fresh water in Britain and probably the most famous after Loch Ness. It is part of the Loch Lomond & the Trossachs National Park, Scotland's first national park. The southern end of the loch is quite flat, but the scenery soon become more mountainous and distinctly wild by the time your reach the north end. To the north-east lie the Trossachs with its Loch Katrine and Loch Array.
Towns and Villages
Balloch -- located at the southern end of Loch Lomond, it is the main town on the loch, and can get busy. The TIC / visitors centre called Loch Lomond Shores can help you plan your stay and provide you with close-area maps. Has a railway station.
Luss -- a town without any attractions other than its beautiful appearance and cottages, which are still worth a visit
Rowardennan -- this village makes a great base to explore Ben Lomond
Drymen -- good base for the Conic Hill, also a nice pub (claimed to be the oldest in Scotland) and Buchanan castle.
Tarbet -- a large village near the centre of the west shore with railway station
ScotRail  Local trains run frequently from Glasgow's Queen Street Station to Balloch which is at the end of the line. The station in Balloch is easy to find.
Several trains run daily between Glasgow and Oban or Fort William which stop at Tarbet and Ardlui on the north-west shore as well as Crainlarich in the northern part of the park. These leave from the upper level of Glasgow's Queen Street station.
To enjoy the nature, it is best to go by bike. There's a (relatively!) well-maintained and -signposted cycleway from Glasgow to Balloch called National Route 7. A good place to join it is Bells Bridge over the Clyde by the SECC (Scottish Exhibition and Conference Center). It's a fairly flat 20-25 miles.
Buses run along the Loch, but not that frequently (see above). You should check the departure-times first, as timetable are not usually available at the stops. The National Park Authority publishes a timetable of all the buses and ferries  which may be available as a booklet locally.
If you're driving, mind that the road along the northern part of Loch Lomond is pretty narrow for the traffic it has.
Sightseeing trips by boat run from Loch Lomond Shores near Balloch.
There is also a useful passenger ferry between Inverbeg (served by Citylink buses) and Rowardennan (at the foot of Ben Lomond mountain).
Loch Lomond Shores, Balloch, 01389-722406. A gateway center for Loch Lomond with an information center, an educational cinema, shops and a view across the Loch. Lomond Shores Project
Balloch Castle Country Park, Balloch. The first park in the national park with a castle you can visit. The castle itself only has two accessible rooms with only little information about the castle , but rather some educational informations about the national park. It was built in the early 20th century to replace the old castle located further down the hill, and only served as a manor house.
Luss Village Paths, Luss (along the A82). A beautiful town with four walks around town and a sandy beach. A map can be picked up at Balloch TIC or at the Luss Visitor Information.
Cycling is probably the best outdoor activity besides hiking. The Lowland Highland Trail, which is part of the National Cycle Network Route 7 starts from Balloch, going north to Drymen, Aberfoyle, Callander, Strathyre, Lochearnhead and Killin. Free leaflets with a map of the bike trail can be picked up at TICs along the trail. There is another cycle path along the western shores of Loch Lomond. Bikes can be hired at the Loch Lomond Shores (see above).
The Clachan Inn in Drymen - Scotland's oldest registered licensed premises (1734) - nice pub with good food and friendly atmosphere.
There is a small pub, with outside terrace and nearby pier for boat moorings, on the island of Inchmurrin in the centre of the loch. A ferry is available from Midross, on the A82, to the island.
Cameron House Hotel has a marina bar with views over Loch Lomond.
Loch Lomond Youth Hostel, A82 (opposite side of Duck Bay Marina), 0870 004 1136. The hostel could be an attraction itself, as it is an impressive country house. £12.50-14.00/5.00-12.00 (Adult/Child). Opened March-October. SYHA information and booking
Rowardennan Youth Hostel, Rowardennan, 0870 004 1148. Located at the banks of Loch Lomond, it can be used as a base to climb Ben Lomond. £10.50-12.00/5.00-9.00 (Adult/Child). Opened March-October. SYHA information and booking
Dumbarton Castle, Dumbarton (take the A82 south), 01389 732167. It features a castle on top of a rock, overlooking the Firth of Clyde. It was an important royal refuge. Historic Scotland information
Buchanan Castle, Drymen (along the A811, enter through the gate of Buchanan Castle Golf Course). A ruin of an beautiful castle, which cannot be entered (danger of falling apart). Sadly, it is not getting maintained or repaired, but is worth a peek.
Inchmahome Priory, Lake of Menteith (along the A81), 01877 385294. This is the only lake in Scotland. An Augustinian monastery on an island, where Mary Stuart once hid from Henry the VIII. A ferry runs to the island, but is only operated in Summer. Historic Scotland information