We were on the boat opposite way, Amsterdam to Mainz in early July...fantastic...well above any expectations I had...ship was in pristine condition, had a re-hab this year. Nice size, felt like you were a guest on a large party on a large yacht! Never been on a mega-sized cruise ship but don't think I would like that. Service was also excellent, not stuffy or prententious,// no dress-code type like on some large ships, but honest and sincere service by everyone from maids to servers and bar keep. Good food...no pressure to buy anything extra like booze or land trips...did not get booze pass, just bought our wine, beer cocktails as wanted...prices not excessive...decent bottle of wine at dinner 20-30 euro (they keep in for next meal if you don't finish it and return it to you at next dinner)...cocktail 6-8 euros, beers 4-5 euros...no stiff markup. They don't search your bags so bringing liquor should be no problem, but would only be for in room use...would not flaunt it in public areas. My only complaint/regret on the trip was we had lower level room with only sealed window..not bad, but would have preferred room with sliding window to get air...A/C was great but a little weak and stuffy in room...though we were in hotter weather that I am sure you will find if you are going soon.?
Did not take any of the pre-arranged excursions..with boat typically docking right in town, it was easy to do your own thing, which I prefer personally...Mannheim was the only exception where it docked in more of an industrial port area, away from town...There is definitely time for Heidelberg (not much in Mannheim to see)...we did our own excursion and walked to a tram stop..took tram 2 stops to train station, and then got train to Heidelberg...express trains can make the journey in like 15-20 minutes it seemed, but trains with stops will be longer..got a combined city bus, funicular up the hill to castle, and castle admission in one ticket from the tourist office in front of main train station in Heidelberg..was not time to go to University AND castle...wonderful views from castle top......I am sure if you are on arranged tour you will have more time, but no guarantees...If you do it on your own, I would recommend using a taxi from boat in Mannheim to the train station to save time and chance getting lost which loses time.