Scavenger Hunt
Here's an idea that's perfect for delivering a gift in a surprising way. For those who don't know, a scavenger hunt is a game where you have to find 'something' by following a series of clues that lead from one to another until, finally, one of them leads you to the final goal, the 'something' you were searching for.
To put this idea into practice, you just need a good knowledge of the place where you plan to carry out the surprise and create a logical sequence of clues that lead from one to the next, until the surprised person finally finds the surprise (or rather, the rest of the surprise, because the scavenger hunt itself is part of it :)).
Let's skip the theory and get to the action. Suppose you do the scavenger hunt at home: you can give the surprised person a note that says 'The first clue is under the place where your dreams rest,' so they look under the pillow and find another note that leads them to the next clue, and so on. If your house seems too small, you can always take advantage of an outing to the countryside to make a large-scale scavenger hunt. As for the difficulty, that's up to you, but remember, it's about surprising, not setting an impossible mission :P
Lastly, as a suggestion to add a little spice to the hunt: include yourselves as one of the clues along the route, and see how they can convince you to reveal the next clue...