I would skip the Nazi Rally Grounds, as that is out of town and takes a bit of time to get to. I really think you only have time for the Altstadt. There is plenty to see and do there. There is Lorenzkirche, an ornate huge old church that was once Catholic but is now evangelical Lutheran. Much of the church was bombed during World War II, but they restored it to its original grandeur.
There is also the Holy Ghost hospital (Heilig-Geist-Spital), which sits on the river. It was built in the 14th century to care for the sick, disabled and elderly citizens of the town. Nearby is the Shöner Brunnen, which means “beautiful fountain”. Because the old slaughter houses and even the hospital dumped very unsanitary things into the river, the town build the fountain to provide clean drinking water to the square. And of course they made it as ornate and beautiful as possible.
I also recommend visiting the toy museum, which is charming and has so many interesting items. I also loved the German National Museum, which houses some original Dürer and Rembrandt paintings, and several pieces by Tilman Riemenschneider, a gifted sculptor from the 15th and 16th century.
And of course, the Albrecht Durer house. Near there is a street called Weissgerbergasse, which is lined with very colorful half-timbered buildings. Definitely make a point of walking through it - it's incredibly charming! Here's a photo of it. https://www.fotocommunity.de/photo/weissgerbergasse-in-nuernberg-abbe-foto/32963877
By the way, I also recommend Literaturhaus for dinner (and breakfast if you don't eat at your hotel). It's very near the hotels by the train station. I ate there 3-4 times during my 4 night stay in Nuremberg and loved it. Wonderful menu with a variety of items (their Wiener Schnitzel is excellent), and a lot of locals eat here. https://www.restaurant-im-literaturhaus.de/