The Hallertau area (Hops for Beer) is just to the northwest of Landshut. Might be something to consider for a country trip.
A longer day trip to the south would be Chiemsee. Could be nice in July at/on the lake. Wasserburg is unique, but not sure I would recommend it based on my short visit. If you keep going fairly far south Berchtesgaden and the National Park would be great, but probably not a day trip. I think the Königssee is one of the most beautiful places in Germany.
Burghausen the city and castle to the southeast I could recommend for a day trip.
I can fully recommend Regensburg too. Walhalla in Donaustauf is not too far away and unique: http://www.walhalla-regensburg.de/english/index.shtml
Heard of the Battle of Landshutte? (1809): http://en.wikipedia.org/wiki/Battle_of_Landshut_%281809%29
Battle of Abensberg: http://en.wikipedia.org/wiki/Battle_of_Abensberg
Bavarians and Napoleon fighting against Austria together...