Aside from the obvious (Johnson, Nixon, and Clinton) were they any serious attempts to impeach a U.S. president?

John Tyler is generally considered the only other President to come seriously close to being impeached. Tyler had been placed on the 1840 ticket with Harrison as a nod to the state's rights branch of the party and with hopes that he would win Virginia for the Whigs. Tyler was never intended to be President, his strict constitutional scruples put him squarely against the National Republican faction of the party which was the majority within the party. It was hoped however that when he became President he might resign (as he had done in the 1830's) or stand aside and allow Clay to pass his legislation. Tyler was willing to do neither but Clay attempted to work with Tyler to pass watered down versions of a new national bank that would give individual state's far more say in the banks affairs ( such as allowing a state to reject allowing a branch within their state) even these extremely watered down versions were vetoed and many Whigs in the house began calls for Tyler's impeachment. Tyler was removed from the Whig party and most of his cabinet (with the notable exception of Daniel Webster) resigned, the Whig dominated house now increasingly called for his impeachment. Whig moderates helped slow the tempo of impeachment efforts and with the Whig losses in the 1842 elections, impeachment was no longer feasible.

Tyler and Johnson are often noted for their similarities. Both were vice presidents who were unpopular with the party when they became President, and both of their impeachments were largely political in nature. While Tyler had done some questionable things (in particular over the Maine boundary dispute) impeachment was a step too far. There was a book written for the Nixon impeachment examining the crimes and controversies of every president through Nixon, I can't recall the name but it maybe worth checking into if someone else can provide the name.