Tips on visiting Zugspitze?

This you tube video is a bit of a “how to” and contains quite a few tips. Have fun!
https://youtu.be/B-iK1XCMiqY

IMHO you should plan to take your bags with you and spend at least a night in the area... there's so much more nearby that's equally interesting and impressive.

Mittenwald itself is stunning and could take a day, as this daytrip guide instructs. Garmisch-Partenkirchen, which is on the train route to both Mittenwald and the Zugspitze, is 1.25 hrs from Munich and is no slouch either. (Page down on the same link above for some Zugspitze info as well.)

This local rail map shows how close these places are. I doubt you could enjoy them fully in just one single day, however.

We rented a car and stayed in Garmisch-Partenkirchen (GP) for two nights. (We were headed from Fussen to Oberammergau, so this was an intermediate stop.) We did not pre-buy tickets for the cog wheel train because we wanted to see what the weather would be like. If it's cloudy or rainy, then the views from the top would be obscured and not worth the time or expense to go up there.
We walked from our hotel to the cogwheel train station (next to the main train station) and then when you're almost at the top, you take the cable car the rest of the way up. We really enjoyed the visit.
The day we were there, the Austria side of the top was closed, but it didn't matter. There were great views all around. Even in summer, it can be cold up there, so we were glad to have brought warm jackets.

Here is the website to find out more about going to Zugspitse

https://zugspitze.de/en

Here is the website for info about traveling by train from Munich

https://zugspitze.de/en/Service-information/Tickets-rates/Combination-ticket-train-mountain-railways

As you can see, the train from Munich leaves very early in the morning, but yes, it can be done by train from Munich for the day.