I am at the moment not able to use the treadmill (sprained ankle) but can say you don't know how much you miss it till its taken away from you!
I used to do a few different things on the treadmill to prevent the boredom setting in:
Running intervals: 30 sec-1min on a 10% gradient at 10km/h followed by 1 min of 3.5%gradient and 10km/h followed by 1min of 5.5% gradient at 6.5km/h. Repeat as many as you can!
Walking uphill with/without 1kg dumbells. Walk at 11-15% gradient at 6.5km/h for 10 mins.
Running at 5.5% gradient 10km/h for 10 mins. Quite intense, if you find it easy, up the gradient or speed.
Walk backwards, then sideways then (if you can!) with your eyes closed- do these on very slow speeds and hold on to the bar or touch the bar with the tips of your fingers, concerntrate on your core taking all the strain, keeping you upright and keeping you walking straight ahead and not into the sides.
I can't run for more then about 15 mins as I get very bored. (I have been able to run for an hour and a half before but needed a loo break about 30 mins in and had to stop after the 1.5 hours as I couldn't hold it anymore!) but the boredom just gets to me. I can say that running outside is easier for running to longer durations and also is more intense as the road you run on is rarely ever flat.