June is beautiful, tho' could be too chilly for swimming. I lived very near Walden pond- lots of nice walking trails. And do spend some time in Concord- there is so much history there, and it's charming. A bit touristy, but the north bridge and minuteman park, orchard house..all worth it. There is a great place for canoe rentals in Concord- nice way to see the sights and avoid the crowds.