A few more fun things you can do with FPGA fabric:
* A temperature sensor - timing some ring oscillators against a reference clock generated off-chip can give you a measure of the temperature of the chip because transistor speed is temperature dependent.
* ADC/power supply monitor - Launching a signal down a TDC at a known interval gives you a time measurement that correlates with power supply voltage (EDIT: to be clear, this is a MUCH stronger correlation than with temperature). Also, ring oscillator speed correlates with power supply voltage. This is also a possible attack vector for side-channel attacks on multi-tenant FPGAs and multi-tenant systems that involve programmable FPGAs.
* An oven/thermostat - Switching transistors on and off creates heat by burning power. This can be hooked up to a temperature sensor (either a synthetic one in FPGA fabric or the one in the device ADC/system monitor) with a PID loop to create a temperature control system. This is actually kind of useful if you want to do analog stuff with reasonable stability.
Also, if the author is here, I would appreciate a link to https://arbitrand.com in the "TRNG" section.