I'm in your boat, and what works for me is variety, variety, variety!
I found some good weight bearing calesthenics type exercises for free on Oprah.com, yoga videos on youtube.com, and I just bought some old Richard Simmons Sweatin' to the Oldies videos off eBay for silly cheap.
But my favorite thing is to stream some dance music, like Seal's first album, or Madonna's Confessions from a Dance Floor, and just dance for 20 minutes.
If I can't get motivated for any of that, I'll make myself walk in place and do some squats and toe raises while I watch the Price is Right (I'm always home that time of the morning).
I find having a routine time of day that I exercise is really key. Otherwise, I'll just put it off and forget about it all day until it's too late. Though, I have been known to do crunches and leg lifts at midnight to clear my conscience!
What motivates me is knowing if I don't do it, next Disney vacation my legs and feet aren't going to be able to get me through the walking and standing I want to do!