Don't let the cat out of the bag! It is thought that this saying comes from the Middle Ages where people would buy pigs at a market and carry them home in a bag. Unscrupulous merchants sometimes replaced the pig with a cat! But this scheme relied on the customer not letting the cat out of the bag before they got home!
Planning a surprise party for someone doesn't have to be stressful - and at least you have got finding the surprise birthday party invitations out of the way now! The key is to make sure that everyone knows to keep things a secret. The last thing you need is your venue phoning the birthday boy (or girl) and asking them a question. Make sure everyone is clear that they need to deal directly with you. Providing them with a mobile phone number is a good way to deal with this - just make sure you keep your phone with you!
In planning this type of party I have found that the hardest part is to get the recipient to the venue without them suspecting anything! Try to think up a good plan well beforehand. If your party recipient has any hobbies or social groups that they belong to, then ask if they can pretend to be holding a function on the big day.
You will also need a way to keep all the party supplies and gifts hidden. If you are planning your own menu, then food might be another stumbling block. You are going to need to find a way to bake, store and transport the food without your guest of honor finding out about it. Ask a friend to help with this - or if you can afford it, use outside caterers. If you are still at the planning stage and looking for recipes, then the foodnetwork have lots of recipe ideas.
Other areas where you may need to plan ahead are:
Guest lists - who needs to be sent a surprise birthday party invitation?
Entertainment or music for the party
What table-where or serving dishes will you use. Do you have plenty?
Seating arrangements if it is a sit-down meal
It makes sense to write everything down - this will really help you remember what has been done, and what is left to do. But make sure you keep that notebook hidden!
And lastly, try to have fun! That is what this is all about!