What range of weights are you interested in, @gdnovey ? What about form factors for the sensor? What kind of readout would be ideal (analog vs. digital, "heaviness" vs. actual force in pounds/Newtons, etc.)?
I ask all these questions because I'm working on a second wave of modules and want to be sure I cover the emerging needs and have buy-in for what we take the time to build.
In the mean time, the cheap answer is a digital switch; the more advanced answer is a force-sensitive voltage divider (something like @williamaadams mentioned + a single fixed resistor) connected to one of Tessel's ADCs, which would give a rough "heaviness" metric.