If you are looking to use points for Marriott stays, unfortunately there are not many choices. When in Rome the Marriott Grand Hotel Flora was fantastic. I have also stayed at the Courtyard by the Rome Airport, but I wouldn't recommend it if your goal is to see Rome (to far away). As for the rest of the country...In Tuscany your best bets are "Agritourismo's", these are basically there version of Bed & Breakfast places. Usually very reasonable and quaint. In the Italian riviera we found a great deal and a wonderful stay at the Excelsior in Rapallo. In Venice we splurged and stayed at the Hotel Danieli. While it was very nice, I am not sure it was worth the price tag. We visited the Gritti while in Venice which is in the same price range, and it looked impressive.
The only other Marriott's worth staying at in Italy are in Milan and on Capri. Which, if I only had 2 weeks of vacation time in Italy, would not be places I would visit. While Milan is a great City, I have only been there once on business. Capri is very touristy, and less enjoyable than other area's of the country.
If you do find yourself in Tuscany, a visit to Siena is a must. There's an incredible sense or history, a tower that you can climb to watch the sun set over the Alps, an incredible Piaza flanked by restaurants with outdoor seating perfect for people watching - not to mention the breathtaking views you'll take in on the drive there.
I've been to Italy twice and Siena was among my stops both times. Enjoy!
When in the Milan area, please go to Lake Como and enjoy the beauty of the region. It even inspired DaVinci, who spent time in this region of Italy. My favorite small town is Chiavenna, (No Marriott;s), which gives you the feel of Italy 500 years ago.
This is a great trip or stop on your way to St. Moritz and on to Zurich across the Julian Pass.