Our last day is in Munich after 14 days in Europe.

Depends on your interests. There's Nymphenberg palace or the Residenz, if palaces are your thing. There are several great Art Museums and the Egyptian Museum. Or the Deutches Museum. Or if you want to be outdoors, there's the Englisher Garden. Or you could follow Rick's walking tour through the Altstadt.

All of these are easily done from a hotel in the Altstadt, either by walking or public transportation. From Marienplatz , it's only 35-45 min by Sbahn to the airport.

"Is there anything of interest that's not central to Munich?"

It seems like you either don't want to be in Munich, or you think there's nothing of interest there. It's actually a city with a lot of variety. So the only thing you saw was Dachau Concentration Camp? There's a lot to see both outside and inside Munich, but to me it makes sense to stay in Munich and to see something in Munich. Not sure what your family is interested in. This family enjoyed Nymphenburg Palace:

http://www.theworldisabook.com/16991/munich-with-kids-nymphenburg-palace/
http://www.schloss-nymphenburg.de/englisch/palace/index.htm

But maybe you're into art museums, BMWs, or something else.

http://www.munich-touristinfo.de/visits.htm
http://www.munich-touristinfo.de/museums.htm
http://www.munich-touristinfo.de/Sights.htm

To reach the airport by train from Munich for your flight home, try the airport-city day ticket:
http://www.mvv-muenchen.de/en/tickets-fares/tickets/day-tickets/airport-city-day-ticket/

Our last day in Munich we were a little "museum and church" 'ed out so we spent the morning on the grounds of Nymphenberg Palace and the afternoon in the English Gardens (beer garden by the Chinese tower). We took public transportation for both along with a lot of walking and people watching. It turned out to be one of our most relaxing and fun days.

My favorite memory of Munich was walking around the English Garden and having a drink at the Chinese Tower and just people watching. I wished I had spent more time in Munich. I fell in love my short time there. The next time I go back I'm getting a bike and want to ride, sit in cafes, exercise in the garden and pretend I'm a local. Fabulous city.

should we try to be nearer the airport? Is there anything of interest that's not central to Munich?

I don't think it is necessary to be nearer the airport. Unless you stay at a pricey hotel right at the airport, you will spend just about as much time getting to the airport as from downtown Munich.

That said, you might want to stay in one of the towns nearer the airport for the attraction there as much as a few minutes saved. It's a 20 minute bus ride from Freising to the airport. On the hill above Freising you have Weihenstephen, the world's oldest brewery. You can spend you last night enjoying a traditional Bavarian dinner and bier under the shade trees at their Biergarten.