I'd say glacier is probably the harder of the two. I dunno if the caverns freeze, but I'm pretty sure the water there doesn't evaporate, so I don't know if a vanilla desert would be too crazy hard. I've never done a glacier either, could be interesting.
Do aquifers freeze if it's cold enough?
I'd like a magma pipe if you guys have no objections. I never build forts with magma pipes.
As one further suggestion, I dunno how you guys feel about drinking, but we could do a version of Drunk Fort.