Erm, I'm a bit late to post regarding this trip I know, but for future reference:
I went up Ben Lui in February this year, had a not stupidly early start, a muddy walk up to the base of the hill followed by a steep but not boring or long ascent to the top, with a bit of a ridge section to finish which was not exposed or difficult, down across a snow bowl and up another Munro, and back to the car through a different muddy forest and along a bit of railway line before dark.
Our Scotland maps are in the shed and I'm afraid I can't be bothered to go and check which route it was I took, but I'm guessing it was probably a variation of the short/boring one referred to above!
It was not too short nor too long, and not boring! It was great, and I don't often say that about munros
We waded across the river nearest the car park with only those who did not have a good boot/gaiter set up getting their feet wet (and those who took their shoes off, like me). It was not at all treacherous, even though there was a lot of water in the river. It was quite good fun.
Hope this helps for next time! I could check the route on the map at some point if anyone wanted to know.