One of them is Astronomy.
I have presented the Keys to him, via the elementary Geography album (the geography album includes all earth/space sciences, although I've split out astronomy for my online course offering; life science is covered in the Biology album). I also presented the Keys of History to him - and that is where the pieces started coming together. Studying those Ancient Egyptians for about a year now (has it been THAT long!?), which has rabbit-trailed into other realms, including Roman, Egyptian and Greek gods, then into the constellations - and now into practical astronomy.
(Correction: it had been more than a year at the time of writing this... there's an interest that doesn't wind down in this little boy!)
See! All that messiness of elementary does come together ;) Present the keys, give time and resources to learn and explore on one's own (and encouragement and guidance when needed) - and you have a child creating his own astronomy book based on everything he has learned.
And then we discovered the Celestial Almanack. Oh my! It is WONDERFUL!
Samples are available on the website linked below - or please do ask questions!
And please do consider purchasing just one month and see what YOU think! Post back here with your reactions.
(UPDATE: January 2013 - the author of the Celestial Almanack has discontinued this offering, but has other resources available at his website. Please support this homeschooling dad and astronomy enthusiast!)