I use several methods depending on whether I'm traveling alone, if I only want coffee and the type of coffee I want for the trip.

A long long time ago , I was introduced to the Zelco BriskBrew by another girl in my boarding school. It's a dual voltage mini drip coffee maker. It makes a scant 7oz cup at 80 Celsius. Instead of using the brewing basket, we used reusable gold mesh filters that we put directly in the cup for brewing to mimic french press coffee. We bought freshly ground, vacuum-packed coffee via mail order.

Building on that, I now either bring the BriskBrew for a single serving of hot water for either tea or coffee, or a mini dual voltage kettle. For convenience, I use disposable paper filters now that can hold either tea, coffee or some other tisane. The paper filters do absorb a bit of the natural oils in coffee if you are looking for French Press style.

The Zelco BriskBrew's patent ran out and so the unit is discontinued. However, this is practically the same machine but by another brand. I've seen the price fall to the high $20 to low $30 range. This type of dual voltage single serve machine has a safety thermostat and takes quite a bit of time to reset itself for a second cup. I've had success heating more water by adding a second cup of water to the unit gradually during the heating process as the water level in the reservoir decreases.

I have not had the best of luck with immersion heaters. Murphy's Law dictates that when I most desperately need hot water having caught a nasty flu on the road, it will die on me, at 3am, in a snow storm.... So for reliability, I bring the BriskBrew and some tea bags and Via at the very least.