What should I use as a test load to calibrate a UPS?
Heater or halogen lamps are a very good candidate. If you are worried about accuracy, you should find a way to measure the power they actually draw, if you have a multimeter it is not that hard since those are purely resistive loads.

Another candidate is the good old incandescent lamp, you probably would need two or three of those.

The "best" way to do this, as in how a professional would do it, is using a rheostat rated for at least 300 W. Those are basically big chunky variable resistors that can dissipate somewhat high loads, but I guess the price would be too high.