Play outside a lot. Collect lots of nature stuff and start a nature table, basket or shelf. Put a big magnifying glass there. Get a kid size pair of binoculars. We use a pill box, the kind with days of the week, to collect little dead bugs we find.
Buy Magic School Bus DVD's to watch.
Get a blank journal for the child and yourself. Practice drawing something like a tree throughout the year.
Children that age love nature. It is a great time to really connect to science in a pretty fluid way.
The only science kit I like for younger kids is Stepping into Science from Thames and Kosmos.
Raise butterflies, collect some tadpoles from a pond. Observe, Observe, Observe. We did a weather tree one year. That was really fun. I just drew my own. I am on my iPad so I will be back with links.