Great second cruise, Burt! Our first cruise was a 7-day in the Med., then our second cruise was another 7-day in the Med., then our third cruise was a 12-day in the Med. on SPLENDOR (see where this is going???). Splendor is a great ship. You've got a great itinerary. Please note, though, that you'll need a vacation when you get back - those Med. cruises are soooooo port intensive that you'll be worn out.
We did a 12-Med. in May on Millie and did some of the ports you'll be visiting. I'd highly recommend going over to cruisecritic.com and checking out their RCCL meet and mingle - and their European threads as there are groups forming that are looking for others to share private tours. That's what we did this time. If you need more info - contact me at my e-mail address - but give cruisecritic.com a try - their Royal Carib. and Celebrity boards are great at helping folks pull together private tours. As well as giving perspectives on the ship's tours.
Our first time in Venice we purchased the ship's tour (Splendor!) of the waterways of Venice: it gave us a nice perspective of what we wanted to see on our own - and how to get there. It's an easy city to navigate - and if you think you are lost just look for signs pointing toward "San Marco" and/or "Riealto" - two main points in the city. It's just a great city for walking.
Mykonos: ship will probably provide a free shuttle into Mykonos town. Again - a great walking town - can't get lost. I just didn't see a need for a tour there - just find the windmills and "Old Venice" - both areas are together overlooking the Med. - absolutely beautiful place for photos and to enjoy a nice lunch.
Croatia: Beautiful! Unfortunately, I was ill in the morning and had to pass on a tour of the country side we had booked. But we did take the ship's transportation into the old city and while I was unable to climb and walk the walls of the city (darned!) we did enjoy just walking along and enjoying the beautiful scenery. It's just a beautiful stop.
Athens was okay: we took a ship's 1/2 day tour of the city and were disappointed. Spent hour or two in the "plaka" shopping and eating before returning to the ship.
Sorry to see that Santorini isn't on your itinerary: it's to die for! Oia at sunset is just (no words to describe).
You'lll have a great time - and e-mail me if you have other questions.