When the weather is nice we like to go letterboxing. Most of the letterboxes we've found have been on hikes. There are a fair amount hidden in urban areas too, but for the most part we're out in the woods and the dirt. Carrying our stamp and book in my purse works for the urban boxes, not so much for the ones in the woods.
free pattern by Anna of noodlehead on the robert kaufman website. I made the small size and it fits our supplies perfectly with some room to spare for phone and keys. I bet I could even fit a snack or two. The small size is the perfect little bag size. I'm 5'9" and it doesn't look like I stole it from my six year old.