The classic neuropsych Trail Making Test, online and free.
Used by neuropsychologists since 1944. Connect numbers and letters in alternating order: 1, A, 2, B, 3, C, 4, D, 5, E. Sounds simple. The cognitive load of switching between number and letter sequences is what makes it hard.
The Trail Making Test (Part B specifically — the alternating version) is one of the most-validated measures of executive function. It's used to assess attention switching, processing speed, and to detect early signs of cognitive decline. Practicing improves all three.