I've been doing a bunch of lifting and core strength exercises. There are a bunch of them that are new to me, and tough. My favorite pair, though, are not the most challenging, but are somehow really fun. You sit on one of those inflatable balance balls and take your feet off the floor and balance there for a minute. When I first tried this, I kept rolling off the ball, and so it took me a lot of tries before I could balance even for 30 seconds. But this morning I did it for two minutes. To make it harder you bring your hands (which initially I have holding onto the ball right behind or beside my butt) up into a prayer position in front of your chest; to make it really hard you bring them over your head to raise your center of gravity. It's not a major strengthening exercise but your abs are engaged to balance you and for me it's really satisfying to see the amount of time I can stay there increase.
The other one is similar, only for this one you kneel on the ball. Knees are slightly spread, maybe hip distance apart. Start with your hands on the ball, and then as you get your balance raise yourself up and bring your arms in prayer position to your chest, then over your head. This one works your inner thighs and your butt as well as your abs as you make lots of tiny adjustments to stay balanced on the ball. I think it's a little harder than the sitting one.