Uhh...lots of things. Look up the Starr Report in wikipedia. Witness tampering and intimidation of Betty Curry comes to mind. And perjury under oath - it got him disbarred as an attorney in Arkansas. It's there if you look.
'SHOW ME WHERE THE PRESIDENT DID ANYTHING TO BE IMPEACHED': HOUSE MINORITY LEADER KEVIN MCCARTHY; SHOW ME WHAT PRESIDENT; BILL CLINTON DID TO BE IMPEACHED?