"Good luck on your operation" and "Get well soon! We're missing you!" are popular get well wishes. "Hope you are back on your feet soon" is also a general saying that is appropriate in different situations.
Get well wishes express positive feelings of empathy and support, and they often try to cheer up a sick person. They can range from a simple "We're thinking of you" to a more light-hearted "Wishing you a super duper, fast and zippy recovery!" Sick people generally find comfort in knowing that friends and family are around to help, so saying something such as "If there is anything I can do to help, let me know" also works as a get well message.