If we want to stay in the USA, sort of, we'd have to hit Mt. McKinley (Denali) which is 20,320 ft above sea level.
Although it’s not considered a technical climb, Denali is extremely challenging and dangerous. Due to its proximity to the Arctic Circle, climbers must cope with intense cold and severe winter weather. Temperatures plunge to -40°F with winds gusting up to 100 mph. Powerful storms often delay climbs for days, as teams wait, huddled in tents or snow caves, making every effort to save fingers and toes from frostbite.
I think I'll pass on that one, so what is the "easiest" way to get really, really high? Pikes peak is a drive and a short hike to the top, so just about anyone can hit 14,000 feet (2.65 miles above sea level.
Kilimajaro is 19,341 ft above sea level (3.66 miles above sea level). While the climb is far from easy (more people have died climbing Kilimanjaro than Everest)
There are six[10] official climbing routes [11] by which to climb Mt Kilimanjaro, namely: Marangu, Rongai, Lemosho,[12] Shira, Umbwe and Machame. Of all the routes, Machame[13] is by far the most scenic albeit steeper route up the mountain, which can be done in six or seven days.[14] The Rongai is the easiest camping route and the Marangu is also relatively easy, but accommodation is in huts. As a result, this route tends to be very busy, and ascent and descent routes are the same.
Of course, at 19,341 feet we would still be 1.83 miles lower than Mount Everest at 29,029 ft.
UPDATE: I guess there a few peaks over 20,000 ft that are relatively accessible:
Cerro Aconcagua – Argentina: 22,841 ft
Like Kilimanjaro, altitude sickness is the most common danger on this mountain. However, a fit person can take on this challenge and ascend this impressive summit, much like an 11-year-old did in 2007.
Cho Oyu (चोयु) – Nepal/China: 26,906 ft
Probably the pinnacle of any amateur mountaineer’s career is to climb an 8,000-meter peak, or one that is approximately 26,000 feet above sea-level. There are only fourteen of these in the world – including Mount Everest, the tallest mountain on Earth – and all of them lie within the Himalaya and Karakorum Ranges in Asia. If you’re interested in trying your hand at one of the biggest, you probably have the best chance of success on Nepal’s Cho Oyu.
There’s no doubt that this climb is far and away the most difficult on this list, not to mention expensive and time consuming (if you allow 6 to 7 weeks for the entire trip.) Going with a guide is mandatory, as is being in top shape for the climb. However, with excellent preparation, training and a little luck, Cho Oyu is a reasonable attempt – if conditions permit – for a climber who has a several high-altitude climbs under their belt. Celebrate back in Kathmandu with a couple of cold Gurkhas on Thamel Street; whether you make it to the top or not, it’ll be well worth the price of the trip.
I think Tiff and I should try this one before we hit Everest ;-)