I wouldn't want to play with time travel, but I would like to see a close-by supernova happen in real time. Betelgeuse is suppose to go boom in about 50,000 years, and it'll be about as bright as Moon for in the least a couple of days. And I'm not sure, it's late so I don't feel like doing a lot of math, but I know Andromeda is like 2.5e^5 ly across, and I'm guessing a buss is something like 1e^-16 to 1e^-18 ly, so dividing the two it'd be in the ball park of 1e^21 to 1e^23 busses. Let me know if I'm anywhere near correct.