Tihany — nice peninsula with an inner lake and church visible from most of the lake.
Frequent trains from Budapest Déli and Budapest Keleti run down the south side of Lake Balaton. Intercity trains stop at main stations such as Siófok, Balatonföldvár and Fonyód. Slower trains stop at all stations. Allow between 2.5 and 4 hours for the journey, and about 3500HUF (approx €17) for the one way fare (First Class). There are discounts for families (33%) and students (65%). Pensioners go free on trains, if in possession of an EU national ID card or passport. Not all ticket windows at the main terminals accept credit cards so you should check the signs. Many ticket machines have English and German language options. Airconditioning is found only in InterCity carriages,located next to the locomotive, for which a supplement is payable. Timetables can be found at 
Flybalaton airport at Sármellék (SOB) at the south-west end of the lake is about 20 minutes drive from Keszthely and Balatonmáriafürdő. The airport is currently not served by international airlines.
The Balaton Uplands National Park is located about 40 km from Zalaegerszeg, to the north of Lake Balaton, the largest (more than 70 km long) freshwater lake of Central Europe.
Exhitibion Sites: Arboretum Zirc, House of Forests Bakonybél, Lóczy Cave Balatonfüred, Hegyestű Geological Exhibition Site Monoszló, Manor Salföld, Lake Cave of Tapolca, Kotsy Watermill Zalaszántó, Matula Hut and István Fekete Memorial Site Diás Island, Folk House Vörs, Buffalo Reserve Kápolnapuszta, Csodabogyós Cave Balatonederics, Kőlik Cave of Szentgál,Disused Sand Quarry Várpalota
Hiking in the northern hills.
Exploring wine cellars. A well worn trail is the climb up Badacsony Hill, which is peppered with small wine growers selling their wines by the glass and bottle. Much of it is mediocre but you may find some gems. Treat it like a pub crawl and have fun. A wine trail leaflet can be downloaded here: 
Cross-Balaton Swim Contest. In 2013 it will be held on 6 July(Saturday), with the first people starting at 0700 and the last starters at 1330. In 2012 over 10,000 completed the swim of 5.2km, with an avergare time of 2 hours 41 minutes. The youngest swimmers were 8 years old and the oldest 82. The race goes from Révfülop on the north shore to Balatonboglár. It may be cancelled in case of bad weather, so better check weather forecasts in advance. If you're taking part you can get discounts on the ferry back. The organisers transport your clothes across, so you can change when you arrive.
Relaxing in thermal spas. The two best known are at Héviz  and Csisztapuszta (just south of Fonyód).
Sailing. There are several annual sailing races for those prefering competition. For boat rental Fonyod, Balatonlelle and Balatonfüred are the best places. A 8m yacht from about HUF 70,000/day.
Swimming. Summer water temperatures range from about 16C to the mid 30s; because the lake is shallow, yesterday's air temperature will be a good guide to the warmth of today's water. There are many places to go down waterslides and just take in a bit of shallow bathing, but be aware that the shallow banks of Balaton are very murky. You can scarcely avoid getting silt everywhere. Beaches on the north shore tend to have more facilities, but have an entrance fee.The shore here tends to be rocky and the water deepens rapidly to a mximum of about 14 metres. Most beaches on the southern shore are free, though large sections of the shore are privately owned by hotels and campsites. The lake shelves slowly, so you can walk out for 500m or more before you are out of your depth, so it's very safe for children. Large free beaches are at Zamárdi, Balatonboglár, Fonyód and Balatonfenyves. There are many hundreds of access points to the shore, so you are unlikely to have to walk far to find a pleasant place to swim.
Since the lake is shallow, summer storms can create very large waves, and people have drowned in most years. A system of warning beacons around the lake alerts bathers to expected strong winds and storms: 30 flashes/minute, colour white/yellow means winds 40-60 km/h, and you should move close to the shore. 60 flashes red/white means winds over 60 km/h. Time to get out quickly!
Kite surfing. On windy days this is an exciting sport, but it takes a few days to learn to stop falling off. Needs a reasonable level of fitness. Centres are Balatonboglár and Balatonfenyves.
Buy local wine of course!
With a vast lake, fish is a good option: Fogas (Pike-perch or Zander) is a white fleshed fish with delicate flavour; Keszeg (bream) is good roasted whole, Ponty (Carp) and sometimes Harcsa (catfish) is the base for halászlé (spicy fish soup), and you'll also find sullo (young fogas) on the menu. Game is widely found in the Badacsony hills and the Nagyberek (near Balatonfenyves), so it's worth looking out for pheasant (Fácán) and venison (szarvas) on the menu.
One kind of local food is lángos, a bread-like pastry usually sold on beaches. It is offered with different toppings like cheese, sour cream, and ham, or often plain just with garlic-sauce on top. Another common "beach food" is cooked corn on the cob (főtt kukorica), grilled sausages (sült kolbász) and of course, ice cream (fagylalt, or more colloquially, fagyi).
The hills north from Balaton are know for their wine. The best you can do is going for a wine spotting tour.
Lake Balaton has been the holiday playground of ordinary Hungarians for over a century, and most of the accommodation is in private houses and apartments rented by the owners, often through agencies. All along the road there are signs for "Tourist information" which are really agents for accommodation and tours, rather than the official tourist offices (called Tourinform). Many visitors arrive without booking and can usually find rooms for around Euro 15 per person/night. Costs for a house with 3 bedrooms can run from about Euro300/week to over 1,000 (luxury house right on lake shore).
It's worth to book the accommodation before arriving because in July and August most of the places are totally full and it is hard to find a free room.
High season for Hungarians is the school holidays, from mid-June to the end of August, but many people go home by St Stephen's Day (20 August). The whole place becomes very much quieter by mid-August, and you can easily find accommodation. On the downside, lakeside bars and restaurants also close for the season.