Peyton can still contribute to football without having to ever play again. He'd make a great QB coach at least somewhere. Tom Brady's post football future is as a talking head or in the broadcast booth, but Peyton Manning's future is in coaching. He should learn from Brett Favre, Brett stayed in football too long & now he is suffering from memory issues(early onset dementia?). Peyton will be a complete cripple if he doesn't retire now.
Edit: If the Giants ever wanted to move on from Eli Manning, I could see the Broncos going after him. The Broncos have a well established history of going with QB's in their late 30's(Charley Johnson, Craig Morton, John Elway when he finally won his two Super Bowls, & Peyton Manning), so going after Eli Manning(if he was available) would not be a stretch for them.