Apart from being in reasonably good physical shape, Kilimanjaro requires minimal preparation. Because you’ll be accompanied by porters and a cook, you won’t even have to do any actual physical labor. You must show up, pay, and go up and down the stairs. Almost the entire way up, it is usually snow-free and above freezing.
Aconcagua is much colder and higher (below -20 C at night at final camp), and it necessitates the use of crampons, but it’s still a reasonably quick trip (20 hours one way, not including time spent acclimating) with porters.
Denali, which is somewhat higher than Kilimanjaro and slightly lower than Aconcagua but requires pushing a 50-pound sled up a glacier while carrying a 50-pound backpack every day for 10+ days, is far more challenging.