I usually get a supply of $2 bills (new ones from the bank) and use them for tipping room service and others. They don't usually see those bills and they often tell me they are going to save them. Just something a little different.
You don't mention what cruise line. Auto tipping (add to credit card) is, to my current knowkledge done on NCL, Carnival, Princess. You can go to the pursar's desk and remove or adjust as you see fit. RCCI and Celebrity are still done last day with envelopes so be sure to set aside the $$.
As for room service, over 9 cruises only done 2x for coffee and and tipped $2 each time. Plan to do it MUCH more on March trip.