While there are nicely design invitations available commercially, to make your celebration extra special you may wish to feature a photograph of your honoree on your invitations and signs.
You may have a nice photo from another event which you could use. For a special touch, you might prefer to take a picture of the honoree holding a sign inviting guests to the event. If the honoree is part of the event planning process and if you know the details of the event at the time the photos are taken the honoree's sign might show all the details. If you do not know the details or don't want to show the honoree the details, just have the honoree hold up a blank sign (or a sign promoting another event or wishing someone else a happy birthday). The details on the sign can easily be removed and new details can be edited in later.
If you are organizing a surprise party, you may want to have your honoree blindfolded in the photos, possibly holding a sign inviting everyone to the event. Of course, having the blindfold say “Happy Birthday” is a nice finishing touch. You can even keep the purpose of the photo a secret from the honoree by blindfolding them first with a plain blindfold and then "double blindfolding" them with the birthday blindfold to play a game at another event.
This opportunity may not come up often, so plan early!!!
For a photo background, you might have the honoree in front of a favorite place, in front of the venue where the celebration will take place or else against a plain background or a festive background. The simplest way to do this might be to position the honoree in front of the background before taking the photo. If the venue is a surprise or far away, another alternative is to place the honoree in front of a plain background (hopefully one which contrasts with their hair color, clothing, etc) for the photo and then replacing the plain background in the editing process.
Instead of having the honoree holding a sign with the details, you could have them pointing to a poster or white board containing all of the details.
If you are organizing a surprise party, you might also include Shhh!!! This is a surprise party. Please keep me blissfully uninformed or similar wording on your invitation.