I'm surprised nobody has posted about finding the speed of light with a chocolate bar and a microwave, because I find that absolutely mindblowing.

The basic experiment is to take the turntable out of the microwave and put in the chocolate, nuke it for a couple of seconds until part of the chocolate starts melting and then measure the distance between the melting patches. If you have a standard microwave, you'll be on a frequency of 2.45 GHz (you can check this online or in the manual). Multiply the distance between the spots by 2,450,000,000 (or whatever the frequency is) and then by 2 and you will end up with c, to within whatever accuracy you measured the melting spots.

I guess if you were really skeptical you could say that you have no reason to believe that v = f * lambda, or that the manufacturers of microwaves or rulers were colluding to decieve you, but I think this is around the point where you can start claming the evidence of your eyes is decieving you and so on - too skeptical to add anything useful to the discussion.