Oo yes, Peter has reminded me: white wool for the clouds, too!
It's a shame you can't make it rain indoors. Or can you? Does rain fall from the clouds, or just from some point above the max height at which you can build things?
(There's a limit to how many mobs are active at one time, and wool doesn't re-grow once removed, so get a pack of wolves and a bow and arrows and go hunting.)
Too late, I'm home now. I did collect... uh, 4 buckets of water, plus sugar, plus red flowers, but can't build much out of moulded plastic and 12 litres of Coke. :(