Drive from Neuschwanstein Castle to Salzburg for a night
 

Looking for things to do. Leaving castle and heading to Salzburg for a night and continuing back up to Munich. Any suggestions would be great

Why not? but only if the main purpose of your trip is to drive.

It's a nice scenic drive with many sights to see on the way.
You cannot do all of them, some will just need a quick stop, others can take up 1-2 hours - but there are (from West to East):

Wieskirche*
Oberammergau
Ettal monastery
Linderhof palace
Lake Sylvenstein*
Open air museum Schliersee (old farmhouses etc)
Kufstein
Reit im Winkl*
Königsee (lake, boat tour)

The more you see on the road, the less time (or no time at all) you'll have in Salzburg, though. The items marked with an asterisk do not require a lot of time to visit, though.
Net drive time along the scenic route without taking into consideration any stops, any lunch breaks, or any visits is around 5 hours.

A faster route which only goes by Wieskirche, Benediktbeuern monastery and Bad Tölz (cute but small old town) and then hits A8 motorway requires some 3 hours of driving (excluding stops or traffic jams on A8).

One your way from Salzburg to Munich, Herrenchiemsee palace would be a "usual suspect" to consider as a stopover.

Where would you stay for a night in Salzburg. Thanks for the info. Very helpful!