Start planning your next day by the shore with this list of the best East Coast beaches. Bethany Beach’s boardwalk includes places to grab a snack, as well as shops selling beach items. (WTOP/Colleen Kelleher) Even if you only have one day to spare, these stretches of sand guarantee a relaxing getaway. A trip to the beach is a favorite leisure activity among Americans — whether it’s within driving distance of home or a flight away. And while tourism in beach and coastal destinations generates billions of dollars each year, free outdoor activities like tanning on the sand, swimming in the ocean and strolling along the boardwalk can make beach…