Activities to do if you can have two grade levels together, such as 2nd and 5th, for 30 minutes or so: math drills with flash cards; with 2 pair of students, rotate reading a story through the group with older kids helping the younger with difficult words; check out library books; do a collage on "sharing"; in short, challenge the younger student a bit more than usual, using the one-on-one help from the older who also gets some review in the process. Pairing can also happen by occasionally sending an individual older student to the lower grade room for a while as part of a "mentoring" program or as a "student teacher".