For each planet that they study they could design either a vehicle that could land on it and take samples of the planet and explain what sort of feet the vehicle would need, the type of material it would need to have in order to withstand the conditions and what sort of items it might find in its samples.
They could research more about gravity, since all planets have it and make models comparing how heavy an everyday item would be on each different planet (for example a bar of soap) and they could make other "bars of soap" from other materials with the correct weight for each planet - it would give their classmates a great understanding of how different the planets are.
They could research about the new Mars rover that is supposed to be launched in the fall, I think it is named Curiosity. Then they could compare it with the other older rovers that have already gone to Mars, in terms of its size, how much stuff it is able to do, the type of things it can do, etc.
They could also look into exoplanets (planets that have been found in other solar systems) - there is tons of brand new stuff coming out about a bunch of new ones recently found
There is also a website that used to be called
galaxy zoo 2 where they can go and look at and help classify galaxies. (It is really cool and my DD8 loved to try and fit the galaxies into a category when she was in grade 1.) It is "real" science that is being done as we speak too, which gives it more of a draw for some kids.
I'm sure I can think of more if you need them.

We are space nuts here!