Have a long walk home after work? Try to estimate the mass of the earth in your head, using "back of the envelope" estimates and conversions you already know.
When you come up with an answer, Google a couple places that have figures for it (you'll find they vary a bit).
Can you come within an order of magnitude? I tried a few years ago, when I was working on a Master's degree, and came right between 2 of the figures I found on the net...