Congratulations!
All your destinations are great and will be beautiful in July, a little hot maybe, but that's OK if you are by the sea.
Oia- Gorgeous, but no beach at the city, you would have to rent a car or take a bus. The city itself is just beautiful, but honestly 10 days would be probably too much, in my opinion. You could however, add in Athens and another island, maybe Paros or Naxos. Day1, leave USa, Day 2 Arrive Athens, Day 3 Ferry to Santorini, Day 4-8 Santorini and Naxos, Day 9 head back to Athens, Day 10 Athens, Day 11 Flight to USA
Naples- very nice, but in my opinion, your other options are better, so I am going to skip this one.
Provence- I LOVE France. Provence is beautiful and there are so many fun towns. You could easily spend 10 days exploring the area. Major drawback though is that the price here is probably the highest you will find in Europe for a beach local.
Spanish coast- COuld be lots of fun but hold out for...
Croatian coast- My top pick. Day 1- Leave USA Day 2- Arrive Dubrovnik, Croatia. Day 3- Dubrovnik Day 4- Dubrovnik, Day 5- day trip to Mostar, Bosnia. Day 6- Dubrovnik, relaxing ferry trip to nearby islands. Day 7- Ferry to Hvar, Day 8- Hvar, Day 9 Ferry to Split, see Split. Day 10, fly to London Day 11- Fly USA
The problem with getting to Croatia is usually you have to go through another stop, like London, to get there, adding another day on your return trip. It is totally worth it though! Inexpensive, beautiful, filled with life!
A Baltic cruise would be great as well, and cost effective, but if you only have 10 days, that probably is not enough time as most of them are 12 night cruises, so with travel you are looking at 14 days.