Going to Munich, what should I not miss seeing?

I still have fond memories of this when my wife and I visted Munich

https://en.wikipedia.org/wiki/BMW_Museum

That trip we visited museums in London and Paris and the BMW museum still stuck in my head. It may have changed since then, that was quite a few years ago.

Well rated at tripadvisor as well

http://www.tripadvisor.com/Attraction_Review-g187309-d190264-Reviews-BMW_Museum-Munich_Bavaria.html

Not sure when you are going but if you are going to Munich at the beginning of May, then the High End is the THE place to go in Munich:
http://www.highendsociety.de/english/highend/highend.php

Munich is also full of record stores:
http://muenchen.cylex.de/branchenbuch-muenchen/L1c1-D_O1c1-M%DCNCHEN-P1c1--N1c1-SCHALLPLATTEN.htm

Also, this is the official site for Munich tourism:
http://www.muenchen.de/int/en/

Rick Steves European travel shows on KCET are among the best. He's been everywhere. Here is his site with advice:http://www.ricksteves.com/

All the best,
Nonoise

Take in the Hofbrauhaus for sure.

I fondly remember my trip to Munich in 2010. Like Albert Porter wrote, you must go to the BMW Museum. It really is fantastic. About a 1/2 day's worth excursion.
The 2nd place I really loved was the Deutches Museum - this is a museum of natural history. Fantastic! Beautiful location as it alongside a river. The museum has a tall tower with massive 10-story tall hygroscope to measure humidity! You can climb all the way to the top & get a great view of the city.
What I also did was travel up & down the subway to all my locations & it was very educational (I can read/write/speak German so it was not too difficult for me but even for non-German speaking folk just using some common-sense like knowing which direction you are headed & holding a subway map you can find your way around very easily).
Of course, you must see the Marienkirche at Marien Platz. In the summer that whole platz (square) will be a magnificient spot - outdoor dining, people milling around, street performers & beautiful weather. Aaaaaah! your trip is making me jealous.....;-)
If you have some time to go a bit outside Munich you can visit the dreaded concentration camp Auschwitz. I did not have time but I would have loved to have visited it.
And, I'm sure that your relatives will guide you well....
Have fun! It's a beautiful city...

Munich in the summer is a fun place! Lots of things to do, don't need a car because of the excellent subway systems (S-bahn and U-bahn). There are a lot of cute but touristy things to do in or around the famous Marienplatz and the less famous Max-Joseph Platz. But, depending on where you stay, you can walk a great deal of Munich and have lots of fun.

Munich has all of the features of a big city in a digestable, small-city form factor. If you have the time, drive or take the train to Neuschwanstein, about 90 minutes away by car. Your kids will love it! Then have lunch in in nearby Fussen (sorry, there should be umlauts over the u in Fussen) overlooking the beautiful Lech River.

If you are a wine guy - unlike the U.S. and much of Europe, beer is relatively expensive in Germany and wine is cheap. There are some really good local German wines. Most winos know about the white wines from the Mosel region, but there are some very good local reds as well, especially from the Franconia region (often labeled as Frankenwein). There is a particularly good wine shop right off of the Marienplatz that specializes in local wines, and the proprietor loves providing tastings and an overview of the German wine culture. Just in case you are interested...

Thank you all for the responses! They are most helpful. We're not going until later this summer (so I'll miss the High End show), but we got a great deal on the RT airfare via Air France that we locked in the dates.

I am a wine and beer lover so I'm looking forward to that too.

Again, thank you everyone. Keep the ideas coming if you think of more.