Start off with craft, using kits from Baker Ross or similar to make something they can wear or wave around - keeps them busy whilst you're waiting for everyone to arrive, as they can just start as they come in.
Do some lively games and then have food - sandwiches, crisps, party rings and buns, then do the cake. Then some more games - maybe quieter ones first, like sleeping lions. If the weather's good , finish off with a treasure hunt in the garden to find sweets. Get them to queue up to collect a party bag, to make it obvious it's time to go.
Make sure you have some prizes but just go for simple things like bouncy balls, rubbers, novelties and hand them out yourself rather than letting them choose.
Remember that you'll have to cut the cake and wrap it to go in party bags - if you can have a second cake to cut up in advance that helps!
I wouldn't put out toys as someone has suggested - you want them to join in, not to be playing with stuff.
Have a running order and include more games than you need. Get everything you need for the games together well in advance so you're not having to search for things as people arrive.
Keep it short - 1.5 hours is plenty at that age.