The Pro Football Hall of Fame quarterback showed up at New Era Field just three days removed from the latest procedure involved in his treatment for oral cancer.
"This is my life," the 58-year-old Kelly said, via the Buffalo News. "This is why I do it. If you don't love it doing it for 31 years, then something is wrong with you. My heart is in it."
Kelly was able to participate in a flag football session with campers.
Last week, Kelly underwent follow-up surgery to insert five implants in his jaw to help reconstruct the area removed because of cancer.
"I knew I'd be here, but I didn't know how much I would participate," Kelly said. "The only part for me [that's hard] to do a lot of is talking. My upper jaw is completely covered in stitches. But I've been hit after games and I was a lot sorer. It's all good. ... It seems like every year I'm sore three or four times for a period of time. It is what it is. I just keep fighting, and hopefully one day I'll be able to walk out of my front door and feel good.
"I've got a lot of things on my plate that I want to do and I can't do them when I'm in this state. But I'm going to keep working. The Good Lord is the only one who knows what the outcome is going to be for what I'm about to do, for what I'm about to go through. I still have a few more surgeries to come, and I take it one day at a time."
Kelly was diagnosed with cancer in his jaw in 2013 and underwent surgery. After doctors determined the cancer had spread to his nasal cavities, he underwent additional treatments and had another surgery in March 2014.
In 2016, Kelly was declared cancer-free before the disease returned earlier this year.
During the early 1990s, Kelly led the Bills to four straight Super Bowl appearances. He retired after the 1996 season and was voted to the Pro Football Hall of Fame in his first year of eligibility in 2002.