i would head for london science museum/natural history. I would go more with what will be of interest to your 13 yr old though as you wont have him/her for much longer and the other 2 will be happy to just be out and about. day at the seaside? chessington?(find a friend to share the rides), or just have an open house all summer and let dc13 have friends round,put paddling pool in garden for younger 2.?