Incident Light Meter
Many photographers still carry a light meter; even in the days of high end digital SLRs. If you're packing light you can use a Styrofoam cup over the end of your lens and use your camera's light meter, hold it in place with one of your rubber bands.
Note that cups do come in different thicknesses, so you may want to calibrate yours using the Sunny 16 rule before you leave.
Record the exposure at the subject, then walk back to shoot the picture.