Lake Tanzawa is a dammed up, artificial lake right out in the mountains in Yamakita, Kanagawa, Japan.
You can get there by train from Matsuda station on the Gotemba line and taking the bus. There are a surprisingly decent amount of buses for such an out of the way place. I think this is because there are many hiking trails dotted around.
We went by car from Mishima, which only took an hour. We parked up at the little museum building in the middle of the lake, paid for the use of the lake for canoeing (500 yen), canoed all over the lake for a couple of hours and then had lunch in one of the few restaurants in the little village near the museum building.
On the lake we saw some ducks and swans and there's a waterfall too which we got a really good view of after pulling the canoe up onto the "beach" area in the vicinity and walking up the dry river bed towards the sound of the falling water.
The lake is placid and calm. Its an easy place to practice canoeing if your inexperienced. There's a road all round the lake. One side is for cars, the other for walking cycling etc. There are many fishermen there and hikers. The area is really just mountains and a few houses dotted around.
After we finished canoeing and eating lunch (I had the dam curry) we climbed up the hundred or so steps to the viewing point to see the Mt. Fuji view.
To the north of the lake up the river Kouchi is the Nakagawa Onsen hot spring area. There are many camp sites, hiking paths, some bed and breakfasts, three decent sized hotels and a public hot spring, which we went to. Its right next to the river at a particularly beautiful point. The hot spring is 750 yen for 2 hours. They have soap and shampoo, a little shop selling local goods, drinks machines (including beer), a nice rest area on the second floor with free green tea and massage machines. There's a bus stop not far away too.
For an out of the way, not particularly popular place (Hakone is nearby and all the tourists generally go there) its actually quite nice, if you like the countryside and outdoor activities. I really enjoyed the day, relaxing and beautiful.