I'm not very good with travel sketching - I always bring my sketchbook, pens and paints everywhere I go, but I never get to sit down and paint that much - I haven't reached the stage where I can paint a good watercolour sketch within half an hour. And I don't want to ask my travelling companion to wait for me much longer than that. Still, every little bit counts. So I'll keep bringing my sketchbook with me.
Airports and train stations are a good place for sketching. Airplanes and trains too (if you're interested in challenging perspective, that is). Lots of anonymous people. There is always a risk they will go before you're finished drawing them, though, so the trick is to pick someone who's just sat down - they're likely to be in place for at least ten minutes, as they don't want to lose that precious seat, unless they're fidgety like me, wanting to explore the shops, toilets and eateries.
This is a sketch of a lady that was sitting in Dublin Airport, waiting for her flight's gate to be announced. I started drawing her, and she got up. Dejected, I closed my sketchbook. But just a few moments later, she came back, and sat in the same seat, at the same angle. So I was able to finish my sketch. I loved her high cheekbone.