Simple way to convert expression to a list?

Like J.M. wrote, the question is if the expression is atomic, because Apply doesn't work on atomic expressions. If we disambiguate the two cases we should be fine:

Listify = If[AtomQ[#], {#}, List @@ #]&

Listify[42]
(* {42} *)

Listify[foo[1, 2, 3]]
(* {1, 2, 3} *)

ClearAll[toList]
toList = # /. { _[a__] :> {a}, a_?AtomQ :> {a}} &;

Examples:

list = {2, a, foo[a, b], s + t + u, Rational[1, 3], 
   Complex[5, 6], <|a -> x, b -> y, c -> z|>, DateObject[]};

Grid[Prepend[Transpose[{list, toList /@ list}], 
  Item[#, Background -> LightBlue] & /@ {"expr", "toList@expr"}], 
 Dividers -> All]