Here's a challenging challenge I kinda just made up outta the blue: Perform a trip to Eeloo in a single vessel of your choice carrying equipment to deploy a permanent, self-supplied colony (Kerbal housing, rover(s), drill(s), sensors, all that stuff). Note that I have not succeeded in this myself, but it should be possible.
-Easy-ish version: Deploy the colony on Eeloo.
-Hard-ish version: Deploy a self-sustaining colony on Eeloo, and deploy a satellite into stable Duna orbit on the way.
-Hard hard version: Drop a self-sustaining colony on Eeloo, deploy a satellite into orbit around Duna on the way, and return the vessel to Kerbin or a space station orbiting it.
-Turboninja-extraordinary-superwow version: Drop a self-sustaining colony on Eeloo, deploy a satellite into orbit around Duna and one around Jool (Or one of its moons, you choose), drive your vessel on the surface to your desired dropoff destination for the colony, deploy the colony, and last but not least retrieve the satellites placed on the way out and return to Kerbin. All in one vessel.
Good luck.