Family traveling to Munich for the first time

You don't say how long this great trip is. How many nights?

Munich (München in German) is the capital of Bavaria (Bayern in German), the largest state in Germany. There is lots to see in Munich itself.
Are you intending to stay in Munich all the time and do day trips, or move to other cities? In Munich, or in any other big city, a car is a liability, use the public transport.
There are lots of other places you could visit by train from Munich, Regensburg, Nuremberg (Nürnberg).
And Salzburg, just over the border in Austria, is an easy day trip.

The southern border of Bavaria (with Austria) is the Alps. This is an easy trip south from Munich.
Füssen and it's famous castles (Neuschwanstein and Hohenschwangau), but this needs a connecting bus from the town to the castles.
Garmisch-Partenkirchen, then up to the summit of Zugspitze (Highest mountain in Germany).
Crossing borders is no issue. The Schengen Agreement means they are "open" borders.

For smaller places (and possibly for Füssen and the castles) a car might be good, but with 8 adults you need two cars.

Here is a map of all the passenger rail lines in Bayern/Bavaria so you can see what is possible by train: https://www.bahn.de/p/view/mdb/pv/deutschland_erleben/allgemein/tickets/pdfs_laender-tickets/2015/mdb_190311_streckenkarte_bay_2015.pdf

I suggest you get googling "places to see in Bavaria" for more suggestions.

Are you planning to stay in Munich all the time? A few days in a smaller village would be a welcome change of atmosphere. I once did it somewhere around Oberammergau, and loved it. That would also bring you close to the Austrian border, and would even make a trip through the Dolomites mountain range into Italy possible.

"traveling with a family... to Munich in June"
After you have your bags, go to the MUC airport train station; two "Airport-City Day tickets" will get you all to Munich's main station.

"old / medieval towns"
Regensburg is a UNESCO World Heritage city and an easy day trip by direct train from Munich. 4 adults can share a "Servus Ticket" day pass (€38/4 round trip.) Leave Munich 9:01, arrive Regensburg 10:20, return in the evening. Just buy this ticket from a machine at Munich's main station.

"do some day hikes / outdoorsy things"
Leave Munich and stay in Garmisch-Partenkirchen. 4 adults can share a "Regio-Ticket Werdenfels" day pass (also €38) to get there (buy from a tickte machine again; you can see the coverage area for this ticket HERE.) Be sure to visit MITTENWALD too. To see a little of Austria, you just take a train ride on this same Regio-ticket to Reutte or other Austrian towns on the Ausserfernbahn train route that leaves G-P. Return to G-P on the same route.

You need a couple of books, both by Rick Steves: Europe through the Back Door and his Germany guidebook. You can probably find them at your local library, along with guidebooks by Frommer and Fodor. If you haven't yet read at least one good guidebook with European travel tips, you're behind the 8 ball already. I am traveling to Germany in June, have been to Europe a number of times, and have been reading and planning for six months already. If you don't have hotel reservations, especially in Munich, that needs to be done soon. It's high season and you can't expect that the good places to stay will still be available to you. Just traveling with eight people may present challenges booking accommodations if you want to stay in smaller, local places.
You'll probably want to stick with train transportation wherever you choose to go, so refer to the website www.seat61.com for specifics. Great info there.
Once you do some research, you'll have specific questions and then come back here and use the search block at the top of the page to search, or post new questions. Everyone here is full of great information, but you can't get everything you need via this forum!
Happy reading!

You need a couple of books, both by Rick Steves: [...] his Germany guidebook.

That book offers good practical informations but I won't label it a guidebook in the traditional sense. It's rather a description of selected places with many unnoticed areas in between. For a systematical overview you would need something like the Michelin Green Guide Germany.

Rick Steves' guide to packing is here on his website. This IMO is his greatest contribution to the novice European traveler. With 8, you cannot afford to pack otherwise, whether traveling around in two cars or by train.

I am somewhat less enthusiastic than Celeste about Rick's guide for Germany. It certainly has some helpful information but IMO also tends to...

• recommend destinations that are given over to swarms of international tourists in a very "un-backdoor" fashion.
• reinforce stereotypes about Germany and Germans.
• overemphasize Germany's Nazi period.

I would definitely look into some alternatives. Some of my favorite resources include Fodor's Germany and Earl Steinbicker's Day Trips Germany, chapters of which you can find online. Here is Steinbicker's day trip chapter for Mittenwald, a town I suggested above.

Seat 61 has good information if you will be taking long-distance trips to other countries or across Germany. It is silent however on the part most train travelers struggle with the most - the local transportation issues.

sla -
I completely agree about the Germany book, but it gives the OP somewhere to start. I also noted Frommer and Fodor so hopefully they'll read more than just Rick! After doing a ton of research myself, here, on other online sites, and in a number of guidebooks, I went back to Rick's book and realized how lopsided it is.

ETA: Russ - hope you see this post. Not enthusiastic, just trying to get the OP rolling. :)

We did a circle train route thru Germany and Austria that included some time in Munich. We started and ended in Frankfurt, traveled to Regensburg, Vienna, Salzburg, Augsburg and Heidelberg. So those spots are all doable.

First time visitors usually like to take the Hop on Hop off bus tour in larger cities. You can get tickets for 24 or 48 hours and they take you all over town with comments from guide telling you what you are looking at. You can get off the bus anywhere on their route--look at stuff--then get back on to continue. Gives you a chance to see what you want to go back to see more of.