Right, this is for the legs!
For super-firm thighs, we have two ideas for you:
Sit on your comfy office chair and put your hands on each side. Lift your legs and stretch them out... higher, higher! Once they are parallel with the ground, hold them for 5 to 10 seconds before releasing and breathing out slowly. It's not over yet... but nearly. Repeat the exercise 10 times!
Sit on the front of the chair so that the thighs are not touching the seat. Then, lift your right leg, parallel to the ground like in the first exercise. This time, hold it for 30 seconds.
If you want to strengthen your calves, it's simple! Grabbing a coffee, meeting a colleague or heading for the printer? Do it on tiptoe: the effect is immediate!~
(and ignore all those astonished looks - you will soon get your revenge)