Techniques for safely leaving a bike for 1-2 minutes in a safe-ish place
Firstly, these suggestions are only going to make it slightly harder to ride away with your bike. I've heard the most common thefts involve grabbing the bike or bikes and slinging them into the back of a van, in which case it doesn't make much difference.

But against the casual opportunist/errant kid...


Have a QR parts and remove them - saddle, pedals?, bars?
I use a small velcro strap to make parking easier. This goes around the bars and pulls on the brake lever on when parking, makes it easy to lean it up against a wall without it wobbling. This might stall someone not expecting it.
Derail the chain from the front rings.
Best of all, get one of those super-convenient rear wheel/frame locks as below:



As long as you've got your key with you - some of them stay in the lock when it's unlocked - it's easy. No more excuses!