I am using the Roland Go-Piano for airline travel. It weights around 8 lbs and can run on batteries. It has 61 keys. What I like about it is that it has a width of only 34.6 inch (87.8 cm), thus it fits perfectly into a duffle bag within airline luggage dimensions. And there are no buttons to break off.
Not sure if it has been mentioned already, but some of those keyboards including the Roland Go-Piano can shift octaves. I have used that feature occasionally and it works as long as the overall range of the song fits within those 61 keys. It took me a while to adjust to the shifted middle C, but it works alright.
Cheers,