Chris was played by Dan Frischman from the series Head of the Class (remember fellow Head Of the Class alumnus Brian Robbins was a co-creator while another cast member of that series, Dan Schneider, was a frequent producer).
Schneider himself appears in "Baggin' Saggin' Kel."
In one episode, one of Kenan and Kel's school friends is played by Josh Server.
Nick Cannon (who was a cast member on the newer episodes of All That) appeared in a couple episodes and was credited for warming up the audience in the end credits.
Kenan later went on to be in Saturday Night Live - kind of amusing given that in this show, he was the Straight man to Kel's Funny Man. (However, anyone who has seen All That can tell that Kenan could be funny too.)