Don't forget the lovely Tolumnias we were discussing earlier this week. Very small plants that produce amazingly generous cascading stems of gorgeous flowers. Look for those for sale at Lowe's this month, they're very popular. If I had a windowsill with morning sun, that's what I'd fill it up with.
Beautiful mini-Phals are available for relatively little money, too. I think the small Dens are the best value for repeat bloom, though. Most of the others only bloom once a year, although their flowers last a month or more sometimes.
Not sure about Den Spectabile being small, Ken.. . . all the ones I've seen recently have been pretty huge.