I find a monopod incredibly useful when travelling - fits within cabin baggage restrictions. I strap it onto my small camera bag, and even, as happened last October, my main bag with a tripod in it goes missing, I'm still able to shoot immediately. I use a Manfrotto 680B, for the price it is very good, easily copes with my heavy set-up. Clearly not as good as a tripod, but much easier to work quickly with. Technique critical - try and make yourself the missing two legs of the tripod, and you soon get the hang of it. It makes the difference, for me at least, between getting shots and not.