I think we all know the main reason to play this game is to achieve great things. But what are some ways we can record our achievements? Flags are a good start, but you don't have to come back to put a flag down, which is substantially harder to do. The ability to recover your ships is nice but it doesn't write down what you recovered anywhere so that's no good either. Here's my suggestion then: Allow space ships to get covered in various colors of dirt from being in the atmospheres or landing on other planets. Nothing like returning to kerbin in your reusable space plane covered in green dust from your awesome Jool aerobrake or having your landers be covered in streaks of variously covered dirt, proportional to how well you managed your landing! Or maybe you skidded to a stop in your rover and now it's covered in Duna-dust. I think this would add a lot to the game graphically, immersively, and just overall awesomeness. It's a great incentive to reuse your designs because now you can have a space plane covered with the dust of several different planets, how awesome is that!? Once on your ship the dirt would gradually fade away, maybe leaving just a slight trace of color so your ship doesn't end up looking like a ball of mud.