Swifities in Munich

Do you mean that you don't already have a hotel reservation??? Have a look at Booking.com, and filter for budget and required amenities. Expect prices to be high for the days immediately before and after the concert. How to get to the Olympic Stadium will depend on where your hotel is. But the U3 goes to the Olympiazentrum stop, and then you walk. Or you could take a taxi. There's more information here:

https://www.olympiapark.de/en/the-olympic-park/visitors-information

Activity suggestions? How long will you be there? What are your interests? Under the Explore Europe section of this website, go to the At A Glance section for Munich. There's a long list of sightseeing options.

Check to see if THE Hoff is playing while you there. It could be a nice second show.

I am looking to do the same thing! Would like to be able to walk to the concert. Where should I stay? Meininger seems to be closest, but, sometimes closest isnt best.

I'm afraid the hotel prices will be quite high. I would recommend to book a room immediately. A hotel close to the Olympic Park will certainly help to get to bed quickly after the concert.
The good news is that public transport is good enough to get to a hotel in the city center even after 11 pm.
https://www.mvv-muenchen.de/en/index.html helps to find the best route.

The subway U3 from Olympiazentrum will be crowded after the concert with long waiting times to catch a train. The alternative is to walk through the park to Dachauer street and take Tram line 20 or 21 or call a taxi from there.

I don't know the Meiniger Hotel, but there is nothing of interest around the hotel. To stay near the Olympiapark I would recommend the Leonardo or Mercure Hotel near Leonrodplatz. Here you would have all the necessary infrastructure (restaurants, grocery stores, pharmacies etc) around, but they seem to be booked already for that date.

Edited:
I checked the hotel availability on booking com for the 27.7. The only reasonable hotel under €200 per night that I found is the “Motel One München-Parkstadt Schwabing”.
It's in a business district, but it's still accessible by public transportation after midnight and it's not too far by cab.

Thanks for all of this info so far!

I did book a hotel in the city center through booking.com so I have some flexibility to change if needed.

That is my concern for travel back from the concert as it will be late at night and in a foreign city.

On another note, I was thinking of adding a few days in a different location, accessible by train, before Munich. Considering Garmisch-Partenkirchen or Mittenwald for some natural scenery before seeing the city. Would even be open to the night train if feasible. Any other suggestions!

That is my concern for travel back from the concert as it will be late
at night and in a foreign city.

There is no reason to be worried. You won't be alone and Munich is safe at night.
It is best if the accommodation is within walking distance of a subway station on lines U3 or U2 or streetcar line 21. Use the same route to get to the Olympia Park, then the way back is not unknown.

My husband and I will be there for the concert on the 28th. My understanding is that for such a large event, they will be adding additional direct trains to the city center.

While you can go to Garmisch from Munich, I suggest if you want to add a different place you consider either Salzburg or Nuremberg, both of which are very easy to get to by train, and easy to get around once you're there.

My understanding is that for such a large event, they will be adding
additional direct trains to the city center.

Yes, exactly, but it will still take a while until all visitors have been transported.
You may check following web site for operational messages about Munich public transport.
https://www.mvg.de/dienste/betriebsaenderungen.html

For example that’s an operational message from today. Metallica is playing a concert at the Olympiastation tonight.

Major event at the Olympic Center: Dear passengers, a major event is
taking place today at 18:00 in the Olympiapark. For this reason,
increased passenger volumes and delays are to be expected on lines U3
and 173 from around 16:00 and after the end of the event from 23:00.
Additional trains will be in operation as well as shuttle buses
between Olympiazentrum (U3) > Frankfurter Ring (U2) > Studentenstadt
(U6) after the end of the event. Please also pay attention to train
destination displays and loudspeaker announcements. Your MVG

We have now added two nights in Halstatt and 3 nights in Salzburg. What is the best train for us to take from Munich airport to Hallstatt?

That's going to be a very long day, if it is your arrival day. You would need to take the Ubahn into Munich to (probably) Munich Ost train station. Then take a train, with at least one change along the way. Travel time will be over 5 hours. Then a ferry across the lake. You can find train details on the Deutche`Bahn website.

Hallstatt is a very small town that can be overrun with tourists. If you don't have a place to stay and dining options already in place, it can get hectic. When I was there last September, there were 8 Chinese tour buses and no place to eat. I was on a RS tour so we at least were covered with a place to stay and dinner. Pretty little town for sure-but be prepared.