I like doing planks. That is getting on your elbows and toes, keeping your back straight and holding the position for a give length of time. You can start at say 30secs and work up from there. I generally do 1 min then have a min rest and go again for 3 - 5 reps then do 30 sec and 30 sec rest for 3 - 5.
You just build up your time. Then you can do things like one arm extended to your front and then an arm and a leg extended, or you can walk your legs around from side to side whilst keeping the position with your back straight.
Also any exercise whilst sitting / lying on an exercise ball will help activate your core muscles .
Only As Old As You Feel - Sometimes I Feel VERY Old