1.   Far better is to pull those dumb nails and replace them with hot-zinc-dipped galvanized deck screws or nails.

2.   Flatten these boards by driving long, galvanized deck screws through them into the sheathing or studs behind.

3.   For the creaking, drive an extra deck screw through the boards into each joist, using a power driver.

4.   Loose boards should be secured with galvanized nails or galvanized deck screws.

5.   Secure it with galvanized nails or deck screws.

6.   Try unscrewing the deck screws, if they are, indeed, screws.

7.   Use a long bar clamp to draw the stringers together and fasten to the treads with galvanized deck screws.

8.   Use galvanized deck screws to pull the board flats and secure them to the joists.