One of:

• eyes to see a wider spectrum of light (infrared to gamma-ray)
• ears to increase sensitivity or reduce it close to 0 a will
• nose to smell all dangerous stuff that is odourless (carbon monoxide f.e.)
• muscles not to lose training after a few days (I am not sure that wouldn’t fuck up methabolism)
• have an organ to catch radiowaves (while being capable to switch it off)
• brown fat cells to never feel cold (with added super bonus that they also kill cancer by starving it)
• liver to remove toxins without being easily damaged

The choice is difficult :)