Munich to Salzburg and also to Regensburg- which days?

If you are in Munich for six days, I would do an overnight stay at Salzburg. The town gets incredibly crowded during the day and my favorite time to walk around was after 5pm and before 9am to see the architecture. You can still use your Bayern ticket to take the RER to Salzburg but you can't use them on public transit in Salzburg-I went ahead and bought the Salzburg card for that.

As I recall Munich is pretty dead on Sunday and not a lot is open. Museum can be crowded on weekdays due to school groups and the same with Dachau. I'd take the 11am official tour. If you get to Dachau early you can buy the ticket for the tour look around and then meet with the group at 11am.

Thanks, all. I'm staying in Munich in general for other reasons, family.

So you're saying Sunday is a good day to go to Dachau? Yes, I'm definitely planning on the 11am tour. Thanks!

You can use the Bayern ticket to and from Regensburg and Salzburg, but only on Regional trains (There is no RER in Germany- that's France). Local public transit is only included within Bavaria, so not in Salzburg. I agree that, regardless of the day you choose, an overnight in Salzburg would be preferable.

I heard museums in Munich are free/discounted on Sundays

This is true for almost all state and some municipal museums (the three Pinakotheken, the Glyptothek, the Bavarian National Museum ... Overview https://www.muenchen.de/en/sights/museums/free-museums-munich-here-you-have-free-admission), where admission on Sunday costs only €1.
Unfortunately, the most popular museum, the Technical Museum (Deutsches Museum), is excluded.

BTW

the Bayern ticket

If you are traveling alone and your travel days are within the same calendar month, you should buy a Deutschlandticket instead of several Bayern Tickets. Even two Bayern Tickets are more expensive than one Deutschlandticket *), which is valid for a whole calendar month and is not subject to any restrictions like the BT (“9 AM rule”) - it' s valid around the clock on all local transport lines (regional train, S-Bahn, U-Bahn, bus, tram).

*)
1 BT = €32, valid 1 day within Bavaria
1 DT = €59, valid 1 calendar month within Germany (and to Salzburg Hbf)

Oh thank you. That's very helpful about the Deutschland ticket! I'm arriving the evening of July 30th so will have to pay the 14 euro from the airport but then I should get the Deutschland ticket to start August 1st- it sounds like I could use that for the round trip to Regensburg, round trip to Salzburg, round trip to Dachau, back to the airport the next week, and anything else. That's great! I think I've actually heard of this- I have to make sure to cancel it when I leave, right? That seems to work out great because on July 31st I was going to stay local in Munich, do a walking tour, etc. Thank you!

I have to make sure to cancel it when I leave, right?

I think you can even cancel it immediately after you’ve ordered for August. But there are the Deutschland Ticket experts here who can probably explain it better.

Otherwise, if Sunday is your museum day in Munich, that's perfect. Maybe you'll be lucky and the weather will be great, then you'll have the museums all to yourself. Most museums in Munich are closed on Mondays - so it is the perfect day for you to travel.

I have to make sure to cancel it when I leave, right?

The important thing is that you cancel it before August 10, otherwise it will be automatically extended by one month. If you buy it via an app like HVV Switch (from the Hamburg transport authority), you can cancel it immediatley after buying with one click directly in the app. Another option is to buy the DT through a reseller like Tranzer.com, they cancel it for you immediately upon purchase, so you don't have to think about it, but they want a surcharge of about €3.00. NB. If you use HVV Switch, please note: this is a multi-purpose app that you could also use to rent a car. So you don't have to answer all the superfluous questions like the one about your driver's license. And it's best to choose PayPal as your payment method, as they are a bit picky with foreign credit cards. (If you understand German: the easiest app I know is “Mainzer Mobilität”).

You can also buy the D-ticket or Deutschland ticket on the DB website. I have the app and it will download the ticket onto your mobile phone.

The earliest you can cancel your ticket on most of the websites is the 10th, which is around when you are leaving. Set yourself a reminder.

You can also buy the D-ticket or Deutschland ticket on the DB website.

Unfortunately, this is only possible if you have a giro account with an IBAN at a bank in the EU, because DB sells the DT online only with direct debit (https://www.bahn.de/faq/deutschlandticket-monatskarte-lastschrift : "Online (e.g. at bahn.de): Only direct debit possible.")

The earliest you can cancel your ticket on most of the websites is the 10th

On the contrary: this is the last possible date. The easiest way is to cancel immediately upon receipt of the booking confirmation.

I suggest that you get your D-Ticket on the MVV app. MVV is the Munich Metro System https://www.mvv-muenchen.de/en/index.html/, so you'll want the app for travel planning, routes, times, etc.

The D-Ticket includes travel to Salzburg (very nice of the German's to include their very popular neighbor to the east!). However, it does not include local travel within Salzburg. So, Salzburg has a new promotion for free travel within the Salzburg area https://www.salzburg.info/en/travel-info/arrival-traffic/guest_mobility_ticket.