If you fancy a walk past the south bank towards London Bridge station there is lots to see - and most of it is free!!
Your boys may catch the skateboarders - there is an established patch about 5 minutes walk away from the London Aquariuam.
You could pop into the massively impressive Tate Modern for a walk around, and you'll see the Millennium Bridge. Then you walk past the new Globe theatre, a faithful reproduction of the original. The Oxo tower is good fun, too. Often free exhibitions as well as a lift that takes you right to the top for a great view of London (unless you've seen it already from the top floor of the Tate Modern).
There's Gabriel's wharf - a collection of interesting shops and cafes with outdoor seating and lots of hand-carved rocking animals to sit on. The next thing of interest is probably the pirate ship, 'The Golden Hinde', moored near-ish London Bridge station. You can pay to walk around it. The Clink Prison, 2 minutes walk away, is another possibilty if your boys want to see some old fashioned blood and gore.
Finally,the very futuristic Cynthia's Bar is worth a visit. I visited it last year with a group of boys ranging in age from 18 months to 13 years. Cynthia is a real, life size robot who stands behind the bar mixing your chosen drink. Children are made very welcome ( but check this again if you intend to go - I went on a quiet Sunday afternoon) and there is a large list of non-alcoholic 'cocktails' for them.
For food, I'd second the suggestion of the nearby Cafe Fish.