Scavenger hunt for Birthday

It's my boyfriend's (m, 22) birthday early June and I'm tossing around some ideas on how to surprise him. I'm thinking about a scavenger hunt, but I've never planned one. The end would be a nice dinner at a restaurant. Ideally, the dinner would also include some friends of his. I'd love to hear some ideas if anyone has any. He lives in a very walkable area/city. One idea I'd like to look into is leaving a clue in the public library and possibly other public places. I'm not sure if I should do it with him, if he he should do it alone, or if he should do it with a friend. Thanks :)