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You can swim, kayak or canoe at Hopkinton State Park, where the vendor Boating in Boston also rents out standup paddle boards, pedal boats, windsurf boards, rowboats, and (for season pass holders) sailboats.
With a spray park, water slide, and filtered pond, Springs Brook Park in Bedford is a good place to ride out a heat wave. There’s also a sandy beach, playground, and concession stand.