Hiya Toastedmallow!!
You have picked a great area for your summer holiday - it will be busy but there is a difference between animated and crushed like a grape!
First port of call might be
http://www.rivadelgarda.comwhich is the official website of the local tourist board.
As regards train trips, you can look up details on Italian State Railways website, or on my preferred one
http://www.db.de. From Desenzano you can travel East-West very easily across the plain of Lombardy ... I would recommend day trips to
Milan (Cathedral (Duomo), La Scala Opera House, the shops ....),
Verona (the Arena and the quaint centre ... all that Romeo and Juliet mullarky (her balcony etc) is exactly that ... no one really knows where it was!!! ....
Padua for it's cathedral and the old university - a medic friend of mine visited the old operating theatres where human knowledge of anatomy was pioneered, and of course
Venice. You might however want to spend an extra night there - perhaps the Venice threads might help ... good and cheaper hotels in Mestre and Marghera.
Going up the lake, there's the mountains to enjoy ... although you might get the odd thunderstorm due to the change in air pressure during the afternoon, Bardolino (home to a great tasting wine), Soave is also a close wine-growing area and you can indulge appropriately.
Just a reminder that pick pocketing and petty crime are sadly on the increase in the wealthier parts of Italy so PLEASE take sensible precautions. I would also advise train travel to the big cities rather than hire a car ... not only do the motorways have tolls FAR too frequently, but the standard of driving is creative to say the least and finding a parking place in a town centre is as frequent as winning the lottery!
You will have a wonderful time - but be aware that Lake Garda is much favoured by the Germans, so you may find that the second language is German rather than English, especially on the eastern shore! But all nationalities can mix well on holiday, and I'm sure you will adapt to hearing more German than Italian spoken in the cafes and restaurants!