Apart from the general response of "dreaming" which appeals to me as well, but that looks a bit selfish, i think i could spend the eternity both teaching and learning (dont know which one i like most).
With this I mean sharing my knowledge with other people and get new knowledges from them, not being beside a desk with the feared red pen
When i'm busy in teaching stuff and also when im learning new things i really don't realize time passes, and were it not for phisical needs, I could continue forever
Yeah, keep your brain busy and eternity will look like an instant
PS I hope our Minister of Educations never happens to read this, she might consider it seriously and delaying the day of my retirement forever