I've got like a 10 hour stopover in San Francisco (SFO) and I want to check out the city quickly. How early can I check in my luggage or are there any other solutions for the baggage problem?
For a domestic arrival and transit, your baggage would be checked through to your final destination anyway, and it is not a concern.
For an international arrival and transit, which I believe is what you are concerned with, you will claim your baggage, walk it through Customs, and then follow the Connecting Flights sign to the baggage drop immediately outside the doors of the customs area. You can then leave the airport.