DH travels 2-4 days a month (1-2 2 day trips, occasionally some day trips or longer trips). If he's going to be gone for more than a day, I try to schedule one of the local teens to babysit from 330-530 for me... I'll go up to our room and lay down, go for a walk alone, go to the library and read or just run a few errands. Sometimes I'll have her make DS some pasta, and I'll pick up a little takeout for me as a treat. Having that two hour break makes a big difference.
Also, I'll *splurge* and let DS watch a video - particularly when I make dinner. I'll also break out special toys that I reserve for rough days or rainy indoor days - playdoh, markers, finger paints, musical toys, big blocks, building with the couch cushions, etc. Lastly, if I really need a break, I'll take DS to the library and let him run around and play with the trains and kitchen set there while I read a magazine or book.
The key is to have some things lined up in advance, and to ask yourself what help you will need to be a *single parent* for a few nights.