I might have taken the time machine back to medieval Japan and watched the Tokugawa Shogun take over in 1603. Seeing King John sign Magna Carta would have been a sight as well.
But one thing I'd have really liked is being a Marvel subscriber in February 1966. How awesome would it have been -- after enjoying a decent story on the Inhumans -- turning the page and seeing...
What I wouldn't have given to buy Fantastic Four #48 for only 12 cents! I 'd have bought Thor #126 that month too. Thor v Hercules? What's not to like?!?
(The photo is from my personal copy of Fantastic Four #47)