I agree with finding downtime in small doses. I am with Krista on really enjoying running errands by myself. But it's not just being able to go to the store and not entertain someone at the same time - I get to listen to my (not kid-appropriate!) music on the radio or MP-3 (go ahead and take the music into the store with you!), and I get to stop at Starbucks or Sonic and get a treat for myself.
It's also sometimes just as simple as having the TV remote. We only have one TV in the house, and downtime for me can be getting to watch WHATEVER I WANT! Put the kids to bed, pour a glass of wine or make a coke float (without the kids begging to have their own ice cream!), and watch whatever you want!
And, there is NO rule that says I have to go to bed the same time as Dear Husband. He turns in early, and I get a hot bath, an equally steamy novel (brain candy - nothing serious allowed), and a cold bottle of water. Chances are, the bath water and the drinking water are the same temperature by the time I get out.
Otherwise, if you are really wanting to get out - find a girlfriend and go out for appetizers or find a bar or restaurant with live music (chances are, she needs a break, too). On your own, get your nails or hair done (as my husband says, a good hairdresser is a whole lot cheaper than a therapist). And, I love the idea of spending a couple of hours at the library. Hmm - maybe I'll do that on my next day off.