If it was open and water was flowing, the Trevi Fountain is stunning.

However, they just (as in last Monday, we were there) completely drained it, and it is surrounded by a chicken-wire-ish fence, and are doing repairs and restoration work, so it's not accessible, nor is it really stunning right now.

I have no idea of completion timeframe, but it appeared they were going to do a lot of work on it so I wouldn't expect any time soon.