Kahneman's add one / three exercise
I just came across the same section you referenced above in Kahneman's fascinating book. Like you, I was curious how to actually perform it. You are absolutely right, Add-1 (and Add-3 for that matter) seems relatively trivial if you can see the digits while performing the arithmetic.

So I searched for the author's actual study and found a more detailed description of it summarized in another book of his, Attention and Effort (1973). See page 20, which reveals that the experiment was actually harder than described in Thinking, Fast and Slow because the digits were spoken one at time (as opposed to visually seeing all 4 together), thus each digit had to remembered for several seconds until the arithmetic was required.


  The primary task involved the transformation of a digit string: the subject heard a series of four digits (e.g., 3916) at a rate of one digit/second, and he was instructed to pause for a second, then to respond with a transform of that series (4027), adding 1 to each digit of the original set.


I believe a more accurate way to recreate the original experiment with index cards would be to write only 1 digit on each card.