Our first cruise - loved it, yes the boat is a little tired -but the food was excellent and the staff brilliant.
You really don't need to do the trips if you can't afford them as all the ports are fantastic!
I would however, recommend that you save your money and to the trip when you dock in Trieste in Italy - this is a very tired and neglected town. Apart from one square, it really isn't worth walking round - weeds growing through the cracks in the pavement - castle shut on Mondays so do the trip on this day.