I would play guitar because it's my one true passion. I played for about 5 hours straight yesterday, I can see myself doing that for the rest of eternity.
Personally, I'd strap a saddle to a whale and ride that mofo around, given I have oxygen tanks that will last for eternity... Maybe dream for eternity, lucid if possible
U beat me to it bigtime.. not weird though High sex yeah.. although sex is meant to have a beginning and an end, its all equally good imo. And actually sex is meant to end, cause it has a very happy ending usually.. but without all that, high sex.
Martial arts. Just reconnecting with the body. Becoming one with the universe through constant instinctive action. Getting better faster stronger forever and ever.
Be dead. Anything done for eternity would turn to torture. If I had to pick a living activity I would pick heroin. That would take at least a lifetime to get sick of.