My family and I will be taking a day trip out of Garmisch in June. The plan was to hit Salzburg, but I'm also interested in Innsbruck. Any suggestions on which one might be better than the other?
Salzburg. More interesting attractions in the city itself.
Innsbruck is much closer of course. Salzburg is too far away for a day trip from G-P, even from Munich IMHO, because there's so much to see there.
If you aren't already taking a day trip from G-P to Mittenwald, well, Mittenwald, which is very close to G-P, would be my first choice:
https://hastingshouse.typepad.com/hastings_house_us/travel_in_europe/
Innsbruck since it's just a short distance away. The scenery in Tirol is absolutely breathtaking. We love to get off on the backroads above Innsbruck and go from village to village.
Day trip - Salzburg is too far.
Innsbruck itself and nearby Hall are fun enough, but the valleys south of Innsbruck are really lovely for hiking, etc.
https://www.zillertal.at/en.html
Train to Jenbach east of Innsbruck and up the valley on the Zillertalbahn.
Buses leave from in front of the Innsbruck main station.