Sitting on a firm surface eg edge of the bed, on a stool/chair or even the loo!
Standing up from a sitting position is a very good exercise to strengthen the legs, it can also be an exercise that challenges the balance. So it is one of those exercises to do if you don't have time for a lot of different exercises. This one ticks more than one box!
If possible try to do this exercise without using your arms.
Put your hands on your knees and push down through your hands at the same time as through your feet to come up into standing.
Stand up tall holding your tummy in for a count of 3 before pushing your bottom out, hands back on the knees and sit down slowly.
Repeat the exercise 3-5 times.
If your balance is not too good do the exercise in front of a table so you can put your hands down to steady yourself if necessary.