I'm not sure about online enrichment- although you might try the "Well Trained Mind" community. Most are homeschoolers and have a lot of good ideas for curriculum. In fact they have an entire thread committed to curriculum.
I have found that reading a variety of books (fiction, non-fiction, graphic novels/comics) seems to help. We've recently had good luck engaging DS in a series by listening to the first book on audio.
The key seems to be the "just right" books. Not too difficult or too easy. I've found that when DS tries to read something too difficult, he struggles to remain engaged. Too easy means he's done with the book in a few hours.
If you have a recent DRA or lexile score it can be helpful in guiding. We usually choose 5 to ten points above a recent DRA score. Keeping in mind that "gifted" at least in DS's case means he's going to quickly exceed that number. So the score is only "good" for a short period.
