Also I like to run up and down the stairs to tone my legs. Reverse crunches are great or crunches in generally if u do it right. A misconception with crunches being too easy is because people don't do it the right way, engage their core and breathe the right way. A good rule of thumb is to remember that if u can talk during an excersise, u aren't engaging your muscles and focused.
Planks lol. I have a love hate relationship with planks and anything that engages my core and back. But planks, sit ups, wall sits, lunges. Those are all simple exercise to engage core and back and a little bit of legs