I don't know if this has been thought of before, but it's an app I would appreciate.
I enter a list of tasks into my mobile device, along with a hint as to where I need to do them (e.g., I need to buy a bicycle pump the next time I pass by the cycle shop near my iridologist's clinic). When I next go to my iridologist, my GPS-smart device gives me a friendly reminder to buy the pump.
Places need not be exclusive. "Outside" could be a place that includes all places outside. My device can then show me a list of all the things I can do when I step outside the house.
Urgency and radius may also be parameters. When I step outside, I would appreciate being reminded to do the urgent things first. If I'm driving past a friend's suburb, I would appreciate a reminder to pick up that DVD he'd promised to lend me, if it's not too much of a detour.
I think of HereDo as a short form for "What can I do now that I'm here?"
The number of times I've cursed myself thinking, "I was there just this morning! I should have done XYZ when I was there!" means this will be a very useful app.