Bay seniors and Geezer loyalists know I try hard not to repeat recommendations but sometimes I can’t help myself. If you’ve got grandkids I strongly urge you to get them aboard the 19th century frigates docked in Jack London Square this Saturday and Sunday. As costumed midshipmen (and women) hoist sails and clamber up the masts, you’ll head out to the Bay for a 3-hour mock naval battle including booming, smoking cannons firing across yardarms. Seniors $50. Children 12 and under $40. It only happens once or twice a year.