If it's just the instrument cross check, then give them targeted airspeeds, vsi, turn rates, or any combination. As they improve, give them reversals on any one, for example rate one turn right to 090, then rate one turn left to 270. Then put them together, climb at 110kias, 500fpm in a rate one turn to the right. At 5000' descend at 80kias, 700fpm until 3500' (at which point you go back to 110kias and 500fpm up). At heading 090 change the turn to a rate 1/2 turn to the left, change back to rate 1 right at heading 300.
Keep moving the altitudes and headings for the reversals closer together.
If that doesn't overload them, have them count backwards from 100 in 3's.
For situational awareness, give them something like the above but let them know that you'll be asking them where any one of three airports are wrt to the aircraft. Then ask them where the wind is coming from.
For holds. Just fly them. PC based simulators are great for those.