A great way to make learning fun is by playing scavenger hunts in the library or classroom. You can break the class up into teams (a more rowdy option) or have them work individually. Here are two lists of things to find in a classroom scavenger hunt and a library scavenger hunt. The former is geared towards 2nd and 3rd grade kids, the latter is for 3rd, 4th or 5th grade students.
Here are more scavenger hunt ideas, too!