Ask HN: What dataset could I collect while traveling extensively?

Take a look at the works of Nicholas Felton http://feltron.com/ especially his reports of his years of personal data like http://feltron.com/FAR09.html These might give you ideas.

Gps location for photos - a lot of tools - online and offline - allow you to map your photos on a map which is always a fun way to go through your albums later

One set I've been casually collecting is a sample of feral vs. pet dogs visible on city streets worldwide.

http://mapillary.com/ or http://openstreetview.org/, basically GPS tagged photos or video streams. Note in both cases the datasets will be free to use for non-commercial use but for commercial it's bound to a license.

And OpenStreetMap of course. I recommend the free http://maps.me/ app that works offline. Every restaurant, every phone number helps.