Make a tea mug heater with a USB power source
An electric kettle takes about a minute to boil a litre of water with 3,000 watts. if you've only got less than one-thousandth of that then in all honesty it would take forever but, assuming that heat was produced and kept in the kettle (mug or container) then it will start to warm stuff up.

Goin back to the 3 kW kettle, if it takes a minute to warm a litre of water from 20 ºC to 100 ºC that's an average of 1.333 ºC per second per litre. If it were more like 300 ml (mug size) then it would be about 4.5 ºC per second per litre. Now this has to be divided by about 1000 to represent your actual available power so maybe you could raise the temperature by 1 ºC in about 220 seconds (3 minutes and 40 seconds).

You then have to ask your self "how much does a mug of tea cool every second"? Do a test and you will probably find that it cools a bit quicker than you could warm it so then, you have to find a way of stopping a normal cup of tea from cooling so quickly.

Once you have got the mug (plus insulation and undoubtedly a lid) cooling less than the amount you can warm it with 2.5W you can devise any method you want of burning 2.5W in or around (close proximity to) the tea mug.

The most direct form of heating is better so maybe consider putting some form of heating element into the tea. It doesn't matter what sort of size or shape the resistor it is - if it only warms one small volume of the tea, heat conduction will cause this to equalize throughout the mug.

Also, remember that as the tea warms (hopefully), the constant power will keep it warming some more but, remember this is all a big waste of time without stopping the standard mug of tea losing its heat naturally - do tests and develop insulation techniques.