fly a circuit from TO to LDG without looking at the IAS.
My last 7 students were all flight instructor courses. I did that with all of them and not one had ever done one before and all were all rather freaked out. Of course it's not very hard. The funniest one was my weakest student. We were on final and I could see he was really nervous without the ASI crutch so I asked him what he thought was the airspeed was. The very tentative reply was "65 kts I think" so I took the cover off the ASI and the needle was EXACTLY pointing at 65. I thought the expression of "jaw dropping" was just a saying but that day I saw it actually happen.
Anyway every student I ever taught including the PPL's got this exercise and for strong students I would cover up the altimeter too. Students could stay usually within 50 feet of the standard downwind altitude just by looking at the ground