There are of course dozens of beaches, large and small, in the Lakes Region [1]. On Winni [2], the most popular (and crowded) is Weirs Beach (Rte. 3, 603/524-5046, www.city.laconia.nh.us [3]), which has the advantage of being easily accessible and close to the action. A more pleasant and secluded bask in the sun can be found at Alton Small Swimming Beach (Rte. 28A, 603/875-0109, www.altonparksandrecreation.com [4]) on Alton Bay.
On the eastern side of the lake, avoid Winni entirely in favor of Wentworth State Beach (Rte. 109, Wolfeboro, 603/569-3699, website [5], $4 adults, $2 children ages 6–11, free children under 5) on the site of colonial Governor Wentworth’s former estate on Wentworth Lake. The lake has a bathhouse and picnic area.
Links:
[1] http://www.moon.com/destinations/new-england/new-hampshire/lakes-region
[2] http://www.moon.com/destinations/new-england/new-hampshire/lakes-region/lake-winnipesaukee
[3] http://www.city.laconia.nh.us
[4] http://www.altonparksandrecreation.com
[5] http://www.nhstateparks.com/wentworthbeach.html