Depends how long you have got but the pirate beach sounds like a good idea. Don't know if you have any support in with you but here are some ideas I have used with my Y3 class. You might be able to adapt. Try to include something for all their senses and get them to think about what they would be able to see, hear, smell, taste and touch. You could have a different object/experience on each table in the room and rotate groups of children around the tables. You could have a table where they can touch the sand, letting it run through their fingers/toes and ask them to describe how it feels. On another table you could let them dress as pirates & act out seeing the island in the distance - What can they see? (TA if you have one could record different groups ideas.) There could be gold coins, tresure map and other pirate type objects on another table. Not sure what you could put for them to smell...coconuts?!?! Not sure for taste either - Health and safety may have something to say about salt water!!! Try to get a clip of the sea so chidlren can hear and describe the waves crashing against the shore. Hope some of that helps!