St Ives is beautiful and one of my favourite places in this country.
I didn't enjoy the Tate, a bit too "way out" for my liking, not much actual art to appreciate! However, I still think it's worth a recommendation, just to see it once. The artists do change but it's too modernistic for me - a room filled with balloons. Really???
I love walking round all the side streets going into many of the 50+ art galleries. A wonderful way to pass the time.
If you like tapas, I can highly recommend the Porthmoer Beach Cafe, but be sure to book for the evening as it books up very quickly. It's a good price too.
There are a couple of lovely tea rooms along the harbour, if you are lucky enough to get a seat outside it's a fab spot for people watching. Just loiter for a while or pop into the galleries next door and you'll soon get a seat!
The Barbara Hepworth sculpture garden is good too, you can get combined entry with the Tate.
Other than that, it's wonderful just to walk along the coastal path.