The only survey site I've found that seems to offer a decent return (50-100 miles for 5-10 minutes of my time) is Opinion Miles, which I occasionally use to pick up some extra United Miles. If I responded to every email they send me quickly (so I wouldn't get the dreaded "we have enough responses already" message), I'd probably get a couple hundred a week.
I haven't read the T&C on the shopping portals from major airlines, so I don't know if they're limited to US residents. But that's another way to rack up miles, assuming you regularly spend money on stuff.