I've had a few surprise birthday parties in my life - my 16th, my 30th, my 39th - and each one was totally different and lots of fun once I got over the shock.
Here are a few tips if you are in charge of planning the surprise:
1. Make sure you know the schedule of the guest of honor and that they have no conflicts with your planned surprise party.
2. You'll need a decoy person or diversion activity to keep the party a secret until the last moment.
3. Make sure your invitations clearly state the party is a SURPRISE! List a phone number and email on your invitation order (if they need to call you for any reason) and on your invitations themselves so the guest of honor will not see or have access to your plans and ruin the surprise.
4. If your guests can't "lie" to the guest of honor about their social schedules, hopefully they can avoid the guest of honor until the party or you might want to come up with a decoy story they can tell to keep the secret.
5. Ask your guests to arrive early, at least a half an hour before the guest of honor's timed arrival, and stress that they can't be late or be seen sneaking into the party a step ahead of the guest of honor.
6. Request guests park their cars out of site. You don't need to hear "Why's Joe's car here?"
7. Pray no one will slip-up and blow your surprise. If you've done steps 1-6, you are probably in the clear.
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