Here are a few quick and easy ones:

• Squats will be the most useful. Not exactly at your desk, but you can definitely do it next to your desk.
• Seated leg raises. Keeping your legs straight, hold the side of the chair and raise and lower your legs
• Calf raises. Put your weight on your toes and lift yourself up. You don't need to raise far, just get your heels a couple of centimetres off the ground
• Standing up. Every opportunity you get, stand up. Simple, but useful
• Use the stairs. A lot. Don't go near the lift ever again

These will all help - but nothing office-based will really cover off the fitness requirements fully, so I'd suggest using the gym, swimming or running as further exercise.

Here are a few quick and easy ones that will help your strength:

• Squats will be the most useful. Not exactly at your desk, but you can definitely do it next to your desk.
• Seated leg raises. Keeping your legs straight, hold the side of the chair and raise and lower your legs
• Calf raises. Put your weight on your toes and lift yourself up. You don't need to raise far, just get your heels a couple of centimetres off the ground
• Standing up. Every opportunity you get, stand up. Simple, but useful
• Use the stairs. A lot. Don't go near the lift ever again

These will all help - but nothing office-based will really cover off the fitness requirements fully, especially cardio so I'd suggest using the gym, swimming or running as further exercise.