<p>When I travel I often take my Lowepro Fastpack 250 as my second (under-seat) carry on.<br>
It will hold a DSLR with lens attached (up to 70-200), and a couple of additional lenses.<br>
In the upper "daypack" space I have my noise-cancelling headphones and other personal effects, and it also carries my 15.4" notebook. <br>
There's not much space left for feet, but that's the tradeoff.</p>