Cheap, fun things to do with tiny babies:

*A walk along the Southbank (if the weather's fine)
*Horniman Museum and gardens (free apart from the aquarium), huge park opposite
*Brockwell Park/Battersea Park
*National Gallery, Tate, V&A
*Potter around Borough Market on Fri/Sat (NB lunchtimes best avoided - gets v busy to navigate with a pram)?
*Wander down the King's Rd

My friends are coming to London for a visit this weekend. We all have small babies what could we do that would cost little and mean we are not going too far................HELP