boat
Take a boat trip around Ramsey Island or one of the other coastal islands famous for their seabird colonies. There are many different operators running trips on rigid inflatable boats that can get very close to the shore to give a unique perspective on the marine landscape. Be aware that these trips can get quite wet, although the operators are trying to get around this by installing seats in the middle of the boat so that passengers do not have to sit on the side.
walk
Talk a walk along the coast path. The national park maintain a coastal path that runs along the coast for the entire length of the national park. It's clearly signposted from all the villages and beaches so you can just turn up and walk to the next place along the coast without the need for a map. The walks are not strenuous and there are subsidized bus services that will take you back to your starting point.
beaches
Spend some time on the many sandy beaches in the national park. Pembrokeshire is home to some spectacular sandy beaches that are perfect for families to spend their day. The beaches are universally very clean and there are usually cafes and lavatory facilities available.
surfing
Try some surfing. The beaches at Newgale and Whitesands usually have good surf, especially on windy days. Surf equipment hire is available at both beaches, and several operators run surf and surf kayak lessons.