You have 2 long travel legs. Lübeck-Aschaffenburg can be done on the fast trains on a "Sparpreis" or savings fare ticket from DB for €48/2 adults if you book well in advance. I see Sparpreis tickets for A'burg - Prague for €58/2. The short leg from Hamburg to Lübeck costs €28/2 on the Schleswig Holstein ticket (which you buy at the station.) So if you can schedule yourselves, you stand to pocket a lot of Euros over the cost of railpasses for two.
You can of course use a German railpass - a 3-day pass costs about twice as much total at €284 for two in the twinpass version (cheapest option.) You could buy the railpass at the last minute. The saver fares should be bought ASAP. Trips/tickets after Dec. 12 cannot be booked or even checked until the schedule is finalized - not sure when that will be - but as soon as they are released you'll see cheap savings fare prices and you should grab them before prices go up.
Note that if you overnight in Nuremberg you would need the 4-day pass (€305.) With a savings fare, you CAN do A'burg-Nuremberg-Prague on a single savings fare ticket if you SCHEDULE a stopover. Sample schedule, Dec. 9, with a 16-hr. stopover in Nuremberg:
Lv Aschaffenburg 13:24
Ar Nuremberg 14:59
Lv Nuremberg 7:40
Ar Prague 11:18
DB itinerary page
Nuremberg places of interest, museums, etc - see what interests YOU the most:
Nuremberg TI suggestions