What are the activities in Kiribati?

Kiribati is most famous for its world class flyfishing, great scuba diving, and astounding seabird wildlife. There is currently only one airline with service to the most famous of the country’s islands, Christmas Island (Kiritimati).

Do they have beautiful corals there? I mean Kiribati was known for sinking islands

Yes of course, corals live long. The Coral reefs of Kiribati consists of 32 atolls and one raised coral island, Banaba (Ocean Island), which is an isolated island between Nauru and the Gilbert Islands.