I have a multitude of toys on rotation so there are plenty of 'new' things to pull out if needed. I try to create games that may amuse, like building a chubby house with chairs and blanket, having a picnic inside, baking, play dough (even though i despise the stuff) whatever i can think off. Sometimes if they are really cranky i just hop in the bath with them earlier than usual to pass the time. On a really bad day i'm more relaxed with tv, although i know you don't want to go down road maybe save it for the worst days only?
Would your DS go for quiet time in replacement of a nap? My kids never did but i hear some kids go for it.
I'm lucky to have a big backyard also.