I'm not going to use a chair either... I don't have room for one more piece of equipment and still be able to move in my room... so I am going to use either the step on 5 risers or the high step ~ whatever has just been used in previous exercises and is already set up...
As for the Ab's... in Cathe's cardio rotations for STS she has listed an Ab's workout with each of the STS workouts I believe... I can't remember exactly, but they are in there so check them out I did the Medicine ball one tonite and even though it was the shortest one, I still feel that I got a good workout for my abs!