The Science Museum shop was a good idea, I got a rather neat little microscope (sort of "My First Microscope" !), and some other little odds and sods like magnetic thingies that go "bling bling bling" and luminous sticky stars and whatnot.
I should also have specified small enough to take down to the South West on a train, so a bike was never really an option. The chess set was also a good idea, but I think my brother has been looking forward to digging out his plaster of paris Alice in Wonderland chess set molds, and making a decent set for his daughter, as opposed to the fairly bad ones we made as kids ! That, and she would probably be beating me in short order.
Thanks for all the suggestions.