since 1991 I visited the Bolzano area together with my family every year for at least a week, so I'd like to tell you something about our experiences.
Bolzano is situated at about 250 m above sea level in the Eisack valley, surrounded by mountains. It tends to be very hot in summer, too hot for me. In the month of May however it might still be comfortable. Personally I would not look for an accomodation in Bolzano itself, but would prefer something at a higher sea level with much better and more fresh air.
In May there will be very little tourism and I think it's not even necessary to make a room reservation, because you will probably find free rooms everywhere. In Summer we used to book an appartment for about 35$ per day in Deutschnofen/Nova Ponente, a small and very quiet village at 1350 m above sea level. Very good air, very calm. The homepage is: http://www.gemeinde.deutschnofen.bz.it/ There is no homepage for our appartment available.
For a highlight like your anniversary however I would probably look for something more romantic. Very close to Bolzano, near the little village of Signato (1200 m a.s.l.), you find the Patscheider Hof: http://www.patscheiderhof.com/ We really love this old farmhouse with it's 400 year old Stube and very good food. It used to be recommended by Osterie d'Italia, the SlowFood guide to italian restaurants. I don't know, why they removed it. We will certainly return to this lovely place! They have rooms, but I know the restaurant only.
Another very nice village not far from Truden/Trodena is Radein/Radegno at 1500 m a.s.l. It is a real Virgin Hilltown. You can stay in Radein at the Zirmerhof: http://www.zirmerhof.it/indexuk.html This is a very old farmhouse as well, recommended by Osterie d'Italia as far as I know. They have lovely Stuben. Try the world famous Zirmer tart, a cake made without any flour. My wife has got the recipe of the Zirmer tart and all of our guests love this cake.
For our own 25th anniversary in 14 years we would perhaps choose the hotel Steinbock in Villanders/Villandro. It seems to be more expensive, but has very beautiful and very typical tyrolean Stuben and rooms. The website is: http://www.dorfmann.it/steinbock_1_it.htm This is also a recommendation of Osterie d'Italia. We ate there very well and saw the very nice rooms. The little village has a beautiful cemetery with great panorama of the dolomites.
Now you know our favorite (and most beautiful) restaurants and hotels in the surroundings of Bolzano. We had a great time over there with excellent food and wine and really felt at home.
Regards Johannes Haltern, Germany