Hmm... 10K calorie challenge?

 Well... the intake:

Let's take Hafþór Björnsson's diet. That's about 10.000 kcal :-D

Breakfast:
6 fried eggs, lots of bacon, french toast, and orange juice.

Second breakfast:
Rice, spinach, bison, stock, and an orange
(About 200 g meat, a cup of rice (boiled), 1 tbs butter, and 1 tbs dextrose. Yes. Sugar.)

Elevensies:
the same as 2nd breakfast

Lunch:
the same as 2nd breakfast + carrots and eggs

Afternoon tea:
the same as lunch

Dinner:
Steak and potatoes, carrots, spinach

Supper:
French fries and chicken wings

that's seven meals, and to get to 10.000, each should be about 1500

Some changes to add calories:

Change the breakfast to modified full English

6 slices of bacon, 4 fried eggs - in butter - 2 links of sausages, 1 fried tomato, 3 slices French toast, with 1 tbs orange marmalade on each, washed down with a large cup of milk tea, and a glass of orange juice

About 2000 kcal.

- pastries, the fatter the better. Lots of cream and sugar, like caramel. Chocolate. There's about 300 kcal in a croissant. Eating two croissants with filling, like bacon and cream cheese shouldn't be too hard. Wash it down with juice or a milkshake. Some 600 kcal in cookies and cream milkshake. 

So, what to do? 

It's easier to eat through 10.000 calories than to burn them. A marathon burns about 3,500 kcal. Ironman competition burns about 8.000-10.000 kcal. That's a marathon, 4k swim and 180k biking. The current record is about 8 hours for men and 9 for women. It will likely take 12 hours for an average person. 

Especially if you are eating all that stuff! 

Basically, all the aerobic exercises, like running, biking, rowing, dancing, whatever makes you sweat and breathe harder, burn calories. So - choose 12 hours of your favorite aerobic exercises. If you don't like biking, don't go biking. Just remember it's not leisurely jogging, you have to work to burn those babies. :-D