A google search turns up results for either. The two are seemingly interchangeable. Which is it? Does one see sights or see sites?
Sights is the word used for interesting places to see, often visited by tourists.
The phrase that is normally used is "sights to see."
As you can see, there is a wide range of meanings which cover both words, so they can be used as you pointed out.
E.g; We have seen the site, it's beautiful.
E.g; The Hermitage Museum is one of the most important sights to see for any visitor to St. Petersburg.
See the sights is also an idiom.
1) We plan to visit Paris and see the sights.