Be prepared for quite a few children to be dropped at the party and then collected later (even if you don't know the child or parents). I couldn't believe how many parents just disappeared at ds's 5th birthday party, without even leaving a contact number. It may be worth having a pen and paper available and asking them to leave a mobile number if they aren't staying.
Buy a tub full of haribo for consolation prizes when children are out of the game, the children are much more likely to go and sit out if they get a small prize.
I often play party games with our children at school during golden time. Some of their favourites include:
Corners - you could stick up pictures based on your theme eg Pirate, parrot, treasure and pirate ship.
Musical statues/ bumps
Best dancer competition
Pin the tail on the donkey - again can be themed eg, eye patch on the pirate
Pass the basket with forfits instead of prizes eg, bark like a dog, do an impression of a chicken, dance like a fairy, etc. If a child is reluctant to do their forfit alone all the other children can join in to help them out.
Traffic lights/sleeping lions style game - you can adapt it for your theme eg, climb the rigging, mop the deck, walk the plank, 40 winks etc
I'd also suggest:
Treasure Hunt - chocolate coins are great for a pirate theme and are on offer in most places at the moment now that christmas has gone.
Invite everybody to wear fancy dress, have a fancy dress parade round the room ending with all children lining up for judging (by the birthday boy, a grandparent or have a judging panel perhaps.) You could always have a few accessories to hand to dress up any children who haven't come in a costume.
Craft activity - making party hats is quite quick and easy.
Party boxes instead of free for all with food - makes it much easier to control who gets what (especially if you have anybody with special dietary requirements).
Whatever you decide to do have a great day and hope your ds enjoys every minute!