For Aspern-Essling check meseum schedule & keep expectations low
The "museum" is privately owned and keeps very strange hours/days open.
I enjoyed my drive around the area but beyond getting a feal for the geography and lay of the land there isn't really much to see.
The military museum in Vienna is great. I also suggest planning plenty of time to just walk around the whole Inner Stadt area as it is chock full of interesting places and buildings (many of which are not well know or even marked). The Inner Stadt is increadibly well preserved with its narrow acient streets so it is great to just wander about day or night. The food is excellent also.
I did not make it to Austerlitz or Wagram but I understand there is more to see there (but the Deutsch-Wagram museum too is privately owned and keeps odd hours so check ahead).