Day of American Independence! Seems an appropriate day somehow to accompany our visiting American friends to watch Shakespeare's The Tempest at the Globe.
Roger Allam interprets Prospero superbly; there are beautifully wise moments of detachment and observation about the character as he regards himself playing a role and reflecting on life on the island; it fits with the whole 'Shakespeare writing his last-play' theme. I fully recommend you see it, especially while standing in the pit as a stinkpot groundling. Getting wet by a squeezed out cod piece is only part of the fun.
From there, a slow walk along the river with the kids, Thames-bank combing, hoping to find a Roman coin or two.