Heres a couple of ideas ......
Use resistors inline with the PIR sensor contact and another resistor in the alarm panel - make a potential divider with a regulated voltage as the feed and then use the PIC A2D converter to monitor the voltage - a very effective anti-tamper circuit.
Look at the Dallas iButtons - easily readable by a PIC and incredibly secure. Good for door entry systems, alarm arm/disarm etc
PIC based temperature monitoring device - could be useful as a fire detector
Keypad entry system ?
That should keep you going for a couple of hours