We are going to build an IR Replay circuit. This circuit uses both the IR LED and the IR Sensor (you used this in CIRC15) to construct a circuit that can record and play back infrared signals from any remote control.
A great (and really fun) application of this is the Adafruit TV-B-Gone Kit, a way to turn off those annoying televisions in bars, stores, or doctors offices.
The concept of replaying an infrared signal is also super useful in other areas. Some garage door openers open the garage when a specific IR signal is received. Do some exploring to find out which devices you own are operated by an infrared signal, and take control of them.
Another application would be building an assistive device, a device to assist someone not able to perform everyday actions. You can create a great assistive device by combining this circuit, and the FSR, to make a pedal/press operated remote control for anything operated by IR.
You can even combine this circuit and the relay used in CIRC11 to receive a signal from a nearby remote and control the relay to switch the power on a DC-powered appliance (like a fan or a lamp).