The turtle tour can be booked at the national park office. You may see a turtle laying eggs. Turtle season is only from Jun-early September. Turtles are likely only nesting in Jul-Aug with a few hatchlings still coming out in September. The park will happily sell you tickets out of season and promise turtles. The trip is not cheap. Be aware and do not fall for this. Do not book a turtle tour outside of this time as it is very likely you will not see any. The national park will thoughtfully refund you 10% if this does happen. Overnight stays can be arranged. This office will also provide very detailed information about possible treks and walking opportunities, but be aware the island has plenty of no-go areas due to the large military presence there.