I have found the best thing is to follow their interest. My dd (6) is into space so doing drawings and looking at the stars and reading the stories of the constellations are all downhill things because she is interested.
I know many boys who started fascinated by animals and the same principle applies.
A friend of mine takes her kids on expeditions with a bag of Microscopes and a reference book or two so they can collect leaf types and footprints etc seeds.
Projects like family trees are a good branch to genetics how many people go before us and leads to history.
Loads of bbc bitesize on science, they have games and things too.
I think bridges are great for science inspiration for physics. There are so many aspects to bridges, history, maths, culture, design, problem solving, aqueducts (sp?), viaducts, bridges for animals, pedestrian bridges. building bridges over water, Roman bridges etc.
Chess is a great game for developing analytical thinking although I haven't taught my dd yet.
I would like to give my dd female Science role models to find out about as she gets older.