I have to agree with those who're saying to check multi-city, etc. No matter where you're booking, be it nwa.com or expedia or orbitz, you can come up with some interesting things.

For example, I had to go from ITO to NBO for a conference earlier this month (the big one... all those carbon emissions from flying! ) but I wanted to stop in the UK to visit my family, and in UG to visit a friend. Expedia said $6000 for ITO-NBO RT, but ITO-LHR RT on HA/AA (alas) and LHR-NBO RT on KL/KQ with an overnight layover in EBB on the way there came out to only $1700ish. I had a couple nights in the UK en route; after the conference I did NBO-EBB RT for a few nights of R&R with friends in UG, and I'm now in the midst of a week in the UK on my way back.

Some other cities are easy to get overnights in... easy as in, it's unavoidable. I have to fly through Guangzhou soon. NW 9 gets in after the last flight of the evening to my destination city has left, and NW 10 leaves before the first flight of the morning from my destination has arrived. Bingo, automatic overnight stay in both directions.