I'll give a different opinion. I find Lake George positively schlocky and touristy! I agree with the PP that an Adirondack vacation on a lake is a truly relaxing experience and many of the things they named are great things to do. However, I might skip Lake George except for a day visit if you really are interested in this town specifically and head somewhere like Blue Mountain Lake or Long Lake (or if you are looking for touristy), Lake Placid or Saranac Lake. These are all Adirondack towns that aren't too far from Lake George that have a little less "commercial" feel. Don't get me wrong---I love the touristy things---but I also like to drive away from them.
That being said, that side of the North Woods region has better access to white water rafting. If you are into hiking, check out some of the high peaks challenges. The Adirondack Museum (in Blue Mountain Lake) and the Wildlife Museum (in Tupper Lake) are great places to visit.
You could fill a whole week with the following:
swimming in the lakes
rafting
hiking
canoeing
cavern visits
boating and boating activities (tubing, skiing, etc)
eating ice cream
wandering through touristy towns
museum and great camp tours
a trip to Vermont (Teddy Bear Factory, Ben and Jerry's, etc)
...but mostly just pulling up an adirondack chair, a drink of choice (Saranac Ale or a glass of upstate NY wine would be appropriate), and gaze out at the natural wonder of the mountains and lakes
P.S. We have a cottage in Cranberry Lake (which is all the way on the other side of the forest preserve)---but there is a lot of great camping all over the Adirondacks. As for best time to go, it depends on what you are looking for:
Summer- best time for swimming/boating activities IN the water--mountain lakes are cold--note June is black fly season so go with July or August
Fall- Best time for "leaf looking"--late fall brings out the hunters if you are into that (we're not)
Winter- snow mobiling, skiing, etc.
Spring- (which is really very late April and May)--- not my favorite time of year as it is very wet and muddy---but a great time to marvel at a natural world that is literally "waking up" after a long winter