We came to Echo Lake twice during our visit in New Hampshire. The lake is clean and the kids had a great time trying to catch tadpoles. The water isn't too cold, there are picnic tables, and clean bathrooms and changing areas. My eight year old did complain that the sand wasn't as fine as he would have like it, but it was good enough for the kids to play in for three hours.
The scenery is very pretty and there is plenty of shade.