I had the same intentions yesterday, so I went down to the gym in my apartment complex and took the last treadmill, sandwiched between a bunch of buffer-puffers walking uphill. Come to think of it, it's all very hamster-like.
Unsurprisingly, after a few miles of much appreciated empty-mindedness, I got really, painfully bored.
So, I brought out some tricks I taught myself while I was living in London - a city I adore and miss dearly, but one that abounds in crappy weather conditions.
I have found the key to treadmill happiness to be changing things up and regular intervals. Sometimes I will change the incline every 0.5 miles by 0.5% or 1.0%.
Example: run 1 mile, then bump up the incline by 0.5% each half a mile until you hit 4 miles at 3%, then scale back the same way until you're back to flat. That's 7 miles made a little more entertaining.
I also like to play a pace-pyramid.
Example: Run 1 mile to warm up. Or if you're not yet bored, run 2. Then, increase your speed by one increment (depends on the treadmill, but say, 0.1mph) every 0.25 miles, which is a lap around the track. Keep doing this until you are running at about 75-80% effort, sustain this for 0.25-0.5 miles, then slow it back down gradually. Repeat as desired. This, I have found, happens to be a really effective form of training that is much easier to do on a treadmill versus out in the wild.
Again - just try to stay on it. It might be funny to others, but it's not so pretty to fall off.