Only just catching up on the thread in between panic tidying stints! 
Creme yes I did have a thread on Amsterdam, we've been dreaming of it for ages but couldn't do it - just too expensive. I really think that with this penny challenge thing it could become a reality, and we can do it in term time so that'll be cheaper!
Interesting about the centre parcs, I'll look into that but it'd need to be easy by public transport. One thing we were thinking of, since a colleague told me, was a mini cruise thing where you do one night on the boat, then have a couple of days in Amsterdam, and then go back on the boat for another night. That appeals partly for the adventure and partly because it'll slow down the travel (something that I find exhausting since getting ill). :)
I will look into Berlin as well, a friend just went there. City breaks are basically what we tend to gravitate towards, because of public transport. :)
Another holiday thing, we're going on a cheapie Haven thing in March and we were discussing them with some other home ed families, we were talking about maybe doing a joint one! Like loads of families all going to the same place at the same time. Would be awesome!