Complex sentence: Over the next several years Leffingwell organized several significant shows, including a 20-year retrospective of sculptor John McCracken, a 1987 show by Michael Tracy, an exhibition of George Herms, and the eight-institution-wide biennial.
Split into simpler sentences: Over the next several years Leffingwell organized several significant shows. There he organized an exhibition of George Herms, and the eight-institution-wide biennial.