http://www.cichlid-forum.com/articles/driftwood.php
http://www.liveaquaria.com/PIC/article.cfm?aid=91
There are lots of woods sold for aquarium use - Mopani, Malaysian, Manzanita, and that standard pale driftwood that probably is collected from beaches in the USA. No wood that I've ever gotten sold for aquarium use has had a long period of having a white fungal/mold growth. Almost all of it has been completely clean and other than leaching tannins and needing to be waterlogged, it has been no trouble.
Grapevines sold for reptiles have always rotted for me, making a mess along the way. I have tried thick oak branches and they too, are messy, although twigs seem to do fine. I know they say that in a couple of weeks or so that slime will go away, but I really don't want to wait that long, it doesn't make the water look too great either.
Pine is generally not recommended for aquarium use. I didn't think of Balsa but that could work, you could probably also make something out of popsicle or craft sticks and 100% silicone if you feel like being creative.
The Legos sound like as good an idea as any. I don't think there are any dollhouses small enough to work, usually these are big enough so you can stick your hands inside of them and put itty bitty furniture in them, aren't they?
Maybe try one of those resin Christmas village decorations? It's the right time to buy them, anyway.