That's what's so great about it Lewis is you can do whatever you want, set your own challenges or just build stuff to watch it explode (especially in 0.25 where they've improved the explosion graphics
)
I haven't played much with building aircraft but I did have a bash a few weeks ago at building a rocketplane to do X15 style flights. Fun!
I haven't tried the real Solar system mods Steve, which ones do you use?
And do they tweak the rocket scales too or just the system?
If KSP is too easy for you and you crave realism, you should check out Orbiter.
If you want a challenge do a complete Apollo 11 mission with NASSP where you have to learn all the button layouts for the LM and CSM and even have to learn a lot of the codes for the Apollo Guidance Computer. There's a couple of weeks of my life I'm not getting back...