I would think a tablet along with a wireless keyboard would be the best option if you are looking for something small. A tablet with a data-connection even better. The biggest problem would be a good enough screen to use in the sunlight.
It would really depend on what sort of programming one's trying to do. Perhaps sitting in a serene and quiet outdoor environment would help you formulate an algorithm and maybe quickly prototype it or something along these lines which has more to do with creativity than with writing complex code.
But yes, there's a reason why the new Apple campus will have almost all offices which will have outdoor view :)
And I paired it with an apple bluetooth keyboard. But the display is too weak for sunny days. Even in the shaded parts of the park, it's almost impossible to work. Maybe some phones/phablets/tablets are better suited for this than others?
- Omni 10 HP tablet (mid-class) - Bluetooth KBD - Pen
You can really do some work in various of places. I personally write a lot, thus it helps. Not to mention it can open all websites without a problem and if needed, supports desktop apps.
Drawback is the low visibility under direct sunlight.