How do I keep the students that finish early from getting bored?

This is my first year as an elementary school librarian with no background as a classroom teacher. I made a big rookie mistake, I didn't have any books to read with students to start the year and I had to move my room completely over the summer. I'm still new to the whole classroom management thing and I know I've made lots of mistakes since it's my first year teaching. <br><br>For my first lessons I just did a general library orientation where we went on a tour, talked about the rules, and did a library scavenger hunt. I expected that to be too boring for 3rd-5th and too hard for K-2nd. I was right, the older kids are done super fast and get really bored. The younger kids are having trouble even following the directions. I'm not sure what to do, especially for the older students since they finish so quickly. It's so hard to keep them occupied and engaged for the entire time when they are obviously smarter than the activities I have set out. I feel really bad that they can't do something a little more interesting.