What's your definition of a 'decent restaurant'?
There's an interesting selection of stylish places at St Katharine's Dock, directly across the river from Butler's Wharf:
http://www.myvillage.com/pages/restaurants-stk atherinesdock.htm
However, my favourite restaurant in London is really cheap. It's Bistro 1, on Southampton Street (which runs between Covent Garden Piazza and the Strand). You can get a really good three-course lunch, from a varied menu, for just �7.50!
Menu here:
http://www.bistro1.co.uk/Bistro1%20Lunch%20Men u.pdf
Homepage here:
http://www.bistro1.co.uk/
Just up the road is one of the best places for unwinding on a sunny day. (The forecast is good for Saturday). It's the Edmond J. Safra Fountain Court at Somerset House (on the Strand). Sitting outside, with the fountains playing, surrounded by beautiful architecture, is really relaxing. It might be too early in the year for the outside bar to be in place but you can still get a coffee from the caf� and take it outside.
If you enjoy wine, you might like Vinopolis. It's a little 'touristy' but it's different to most other attractions:
http://www.vinopolis.co.uk/
Vinopolis is just to the west of London Bridge station and close to two historic sites which are ignored by most visitors to London. These are the remains of Winchester Palace:
http://www.english-heritage.org.uk/server/show /conProperty.112
and Southwark Cathedral: