This offer from Time Out London gives you 2 for 1 entry into loads of London's top visitor attractions.
One free entry per paying adult and one voucher per person per transaction, offer on until 30th June at the London Eye and until 31st December on all other attractions.
Prices (2 for 1 on all):-
London Sea life Aquarium £20.70
Madame Tussauds £30
London Eye £19.20
London Dungeon £24.60