Thanks for all the replies so far - many of the suggestions are going on our "do these things very soon" list.
The more we look at it, the more appealing Prague is becoming for this particular trip: proximity, cost, food, sights. Seems like a place you could spend a lot of time in, but still get a pretty good taste of it in three days.
I neglected to mention we have a few days before my work begins and a few days after. The after time coincides with the final weekend of Oktoberfest, so we are already booked in a Munich hotel for those days. We went to Munich and Dachau on our last trip, but it was late January and bitterly cold. That same trip we saw Salzburg and Zurich, as well as Colmar and Strasbourg. Something of a whirlwind tour, but loved it.
It's the before time we're trying to fill now, and it's five days total. Figuring in two days for travel, we have a solid three days to spend wherever.
The Rhine River boat tour is on our bucket list for sure, as are Spain and Croatia. Just not sure we can make them work with the time available.