I was in Vienna some years ago and spent a very hot and memorable, rewarding day cycling across the Aspern-Essling and Wagram battlefields on a hired bicycle. I managed to navigate myself around simply using Ian Castle's 'Aspern & Wagram 1809' Osprey Campaign book taking the route Vienna-Aspern-Essling-Raasdorf-Aderkla-Wagram(lunch)-Baumersdorf-Marksgrafneusiedl (the tower is on private land...)-Vienna. It is a vast battlefield so allow plenty of time.
Things to see:
The monument next to the Aspern Church.
Check the opening times for the museum in Aspern.
The Granary in Essling (musket ball holes in door...).
Raasdorf - stand where Napoleon had his command post and try to get a sense of what command and control was like on such a battlefield.
There is a museum in Wagram and a monument in Bauersdorf by the stream commemorating the attack by the French in that sector.
Not sure of the current status of the tower above Markgrafneusiedl but when I visited in 2000 it was still scarred by shot and shell.
The crypt of Marksgrafneusieldl contains a macabre ossurary containing the remains of hundreds of men who fell during the battle - try to arrange a visit with the church.
Top tip; it was extraordinarily hot when I visited so take plenty of water (shops are scarce on the Marchfeld) and a hat (shade is as scarce as the shops!)
Have a great time.