This place is, as many have already noted, huge and absolutely fascinating. Lake View bills itself as "an outdoor museum" in one of its brochures, and the number of famous folks buried here, gorgeous statuary and cool architecture certainly gives plenty of merit.
Must sees include the Tiffany-designed interior of the Wade Chapel (open April through mid-November) and the Garfield Monument (also open April through mid-November).
We spent the entire day at Lake View and still didn't see everything we wanted to see. It's best to do a little advance research to find out which graves and monuments you want to see--there is a map and brochure you can download from the cemetery's Web site or pick up at the cemetery office. You can also purchase a Lake View Historical Trail booklet for a few bucks at the office or in the small gift shop at the Garfield Memorial-the booklet has maps and locations of various graves with short biographies of the folks buried there.