On the shores of Newfound Lake [1], Wellington State Park (West Shore Rd., Bristol, 603/744-2197, www.nhstateparks.com/wellington.html [2], mid-May–Oct., $4 adults, $2 children 6–17, free children under 6) has a quarter-mile beach along with picnic area and hiking trails.
Mount Sunapee State Park (Rte. 103, Newbury, 603/763-5561, www.nhstateparks.com/sunapee.html [3], Jun.–Sept., $4 adults, $2 children 6–17, free children under 6 ) has a wide, family-style beach on the south end of Sunapee Lake [4]. The beach also has canoe and kayak rentals for a spin around the pond.
Sargents Marina (Rte. 11 & Cooper St., Georges Mills, 603/763-5036, www.sargentsmarina.com [5]) offers pontoon, ski, and row boat rentals ($125–250/day) at the northern tip of the lake.
Links:
[1] http://www.moon.com/destinations/new-england/new-hampshire/lakes-region/western-lakes/newfound-lake
[2] http://www.nhstateparks.com/wellington.html
[3] http://www.nhstateparks.com/sunapee.html
[4] http://www.moon.com/destinations/new-england/new-hampshire/lakes-region/western-lakes/sunapee-lake
[5] http://www.sargentsmarina.com