Well, there is a playstyle called 'Caveman' where you can only play with the parts you can get with No facilities upgraded, no use of Mechjeb, Cheats and other Tools/Mods. So, any Craft is limited to 18 tons, 20 meters in height and 30 Parts. As there is the normal Caveman Challenge (getting all Science unlocked on different hardness levels) I, for myself, took this to another level in trying to Land on Celestial Bodies around the System where its not so easy to get to with this limitations