Yes, I agree w/ visiting the lovely parks in London when the weather is nice. Have a picnic there & bring a ball or other toys/games & just run wild on the grass!

You can also visit Convent Garden which is lovely to stroll around and if you're lucky you might see a performance on.

London Tower Bridge, museums, zoo and Hamleys toy shop are places I would also take my 3.5yr. old son.