Demon was considered class Y. Class K, such as the planet of Mudd's androids, could be inhabited but required structures like pressure domes and artificial habitats. Mars would fall under this category in the Star Trek: Star Charts
Other canonical mentions include class T (includes some gas giants, mentioned in VOY "Good Shepherd"); Class J planets are also gas giants, similar to Saturn and Jupiter. (DS9 "Starship Down"). Class H is for areas generally uninhabitable by human life, although they could be adapted like Class K since the colony under Sheliak threat was on a class H world. Class H environments were primarily desert, whereas class K might have an intolerable atmosphere. Class N, mentioned briefly in "Night Terrors" but only elaborated on in Star Charts
, is a greenhouse environment similar to Venus. Lots of gases and no liquid water. Class L planets, seen in DS9 and VOY, were "marginally" habitable planets with fewer natural resources than a class M, but where survival for a time was not impossible.
also mentions a couple of other categories, including class F (geologically active with no atmosphere) and class P (glaciated with lots of surface ice, such as Andor and Psi 2000). The full list is on Memory Alpha