If you like japanese style noodles etc, the chain Wagamamas is very reasonable and can be found all around central London. Ask pizza are another reliable chain that aren't expensive and do good italian food.

If you buy a day travel card to get around on London Transport you should be able to get money off certain boats trips on the Thames. Ask at the ticket offices on the London Eye pier, Westminster pier or Southwark pier (opposite The Globe).

A good walk is to start at County Hall on the South Bank and walk along the embankment to Tower Bridge. Along the way there are various attractions - from statues to exhibitions, street entertainers, views up and down the river, the Clink Museum, the replica Golden Hind, HMS Belfast for example. Once you get to Tower Bridge you can walk across to the north bank and stroll round the outside of the Tower of London fer nothing.

If you like guided tours, there are various guided theme walks, try

London Ghost Walks

It depends on what you want to do.