Like falling off a rock, I think - although that sounds rather painful. How about, "Easy as pie" - an Americanism I've never understood? No? "A walk in the park" - another Americanism? No, that involves effort.

Got it! Easy as picturing myself rolling about on a pelt bed with Karl Urban (as Eomer, natch) with his long blonde hair flowing over me?

Gotta go for a few minutes now! ;)