A few more suggestions...

The past two posts promoted animal projects.  What if your family is large and bans animals?  Do you live on a large lot or near a wooded area?  Consider a mammal list or bird list.  Here are some common feeder birds. Our family loves the Identiflyers we bought on eBay.  We also like to build bird houses.  What about making bird feeders?  We made them at a science camp by rubbing Crisco on squares of cardboard and dipping them in bird seed, similar to these feeders.  You might consider creating a bird friendly yard.  Would you rather attract butterfly's and hummingbirds?  This site sells the seeds and explains the plants.  I thinks herbs and plants are great to plant over the winter in a diy hydroponics system.  My sister-in-law set up a similar one in her basement.  My tower is from Vertigro.  Once again, there is plenty of science.  Which nutrients?  Will ordinary fertilizer work?  Which timer?  How often to water?  Does the pH matter?  Seeds, seedlings, or plants?  Fun, right?  One more suggestion: Sea Monkeys, also called brine shrimp.  Carolina Biological has tips to raise brine shrimp.  This blogger has gone into brine shrimp production.  I can hear you asking what to do with the brine shrimp.  This lab is designed for advanced biology.  Take a look.  It has suggestions for observations, counts, identification, etc. The kids are less likely to get attached to sea monkeys than, say, a bunny.