Here are a couple of fun projects. Both are high-impedance probes, but the 1st (EDN) is meant to drive a 50-ohm load like a spectrum analyzer and the 2nd (Instructables) is meant to be hooked up to your oscilloscope (High-Z).
I have not built either of these, but I do have the parts for the EDN article. The OPA656 looks to be a tad pricey these days, but maybe someone who works for TI might get us some samples for a class?