Welcome to GFE,
chenelsea33.
May is a good time to visit Europe - it's still cool in the evenings but warm enough during the day for just a t-shirt. Plus many of the places you've chosen will be perfect at that time of year.
For your flights, I'm guessing you've looked into open-jaw flights, ie in to London and out of Barcelona, which is a good thing as you will not need to backtrack in order to catch a flight home.
With regard to your itinerary, a few notes and questions:
London - do you plan to spend any time here or go straight to Oxford? If you plan to stay in London, allow 2-3 full days depending on your interests. 2 nights in Oxford will be a good amount of time to catch up with your friend, walk the city and have a pint or two. It's a pretty and compact city, so one day is good to explore the univerity buildings, Ashmolean Museum (if you're in to it), a walking tour, the covered market or punt on the river.
Paris - taking the Eurostar from London is a great way to quickly and comfortably get to Paris. Book well in advance for a deal on tickets. 5 full days in Paris will allow time to explore the city as well as a half-day (usually all you need) to Versailles. A day trip to Normandy will be a long one (around 15hours), but worth it. There are a few organised tours which will take care of the transport side of things.
Eagle or
WillTravel may have more details on these, but if you can spend a night there it would be better.
Rome - flying to Rome is a good idea as the train journey overnight is a long one (15hours). Make sure you avoid flights from Beauvais airport as it's too far out of Paris and there are plenty of flights to be found from Orly or CDG. 5 full days in Rome will allow plenty of time to sightsee. The Vatican is located in Rome, even though it's a seperate country (no passports needed for border crossings!) and you will find a full day is required if you intend to completely explore the Vatican - the Vatican Museums/Sistine Chapel, St Peter's Basilica (including the Crypts with or without a tour), climbing the dome. If, however, you only want to visit the Sistine Chapel (located inside the Vatican Museum) and St Peter's Basilica, then half a day may be all you require. The lines to enter the Museo Vaticano/Sistine Chapel are always long, particularly from around 8am onwards, so if you're keen, get there just before 8, stand in line (take breakfast with you - coffee can be purchased at cafes nearby - down side streets - and taken away so one of you can stay in the line) and wait until opening time (9am). Then you have a good start to your day. Don't forget to check the Vatican website
www.vatican.va for opening times and days to plan your visit - nothing worse than finding out it's closed or you plan to go the day after a public holiday or closed day as the lines are extra long then.
A day trip to the Abruzzo will again be long (4hours one way), and you may find that you want to stay in a town overnight. If you do decide to to this, I'd suggest travelling in the order of Rome-Sorrento- the Abruzzo - Venice - CT.
Sorrento - reached via Naples, take the Eurostar Italia from Rome to Naples, then follow the signs for the Circumvesuviana regional train which stops in Sorrento, then walk down the hill to the city centre. Pompei is reached on this train service too, so you'll have the chance to see the station on your way through. 2-3 full days in Sorrento will be good. Pompei will need half a day, so start early, visit Pompei (get an audioguide at the entrance), then you'll have the afternoon to explore Sorrento just as the shops are reopening (4pm) after siesta, and have an aperitivo as the sun sets. Capri as a day trip will be great - take the 8.30am ferry across (there are fast and slow ones depending on your price range), then check tides for the best time (ie low tide) to take a boat ride to the Blue Grotto. This can be tricky as there are large boats which take you and around 80-100 others to the Grotto and they will make their presence felt as you arrive on Capri. There are also small wooden boats, docked in the middle of the marina (you'll see them bobbing next to a small hut with a "Motoscarfi" sign) which can be cheaper and will take smaller groups to the Grotto. Then you have to change to tiny row boats to get inside the Blue Grotto. It will be the most expensive 5mins of your life in the Grotto, but well worth it for the experience. You can also take a bus to Anacapri and walk down to the Grotto. There are tours of the island by boat but these will take a good chunk of your day and as the water is still a little chilly even in late May you might not want to do one.
Once you've worked out the best time to visit, you can plan when you can visit the rest of the island - take the funicular to Capri town then a hairy ride in the orange busses up to Anacapri if you feel like it. Wander Capri town and the ruins of Villa San Michele or hike up to Villa Jovis, or just sit and sip a beverage of choice while you look over the sea.
Cinque Terre - 2 full days will allow a nice amount of time for walking, sitting in piazzas, strolling, and generally relaxing as it's a beautiful part of the world to do just that. Stay in Riomaggiore or Vernazza if you can.
Travel to Barcelona - this train journey, from CT along the coast, is pretty, but long, especially if you're not sure where to stop. There are places like San Remo, Nice or Antibes, but if you're not really sold on any of them, I'd suggest flying from Pisa to Barcelona (chekc Iberia Air) and spending the extra time in other parts of Spain, as you've noted.
Andorra is a great country, but a bit off the track if you plan to do all of this by train. It's a great place for skiing in Winter, and Summer offers hiking, etc.
With the suggestions of extra days/overnights in some places, see if these suit your plans better and let us know what you've come up with.
