Plunder Al Capone’s vault, before it’s opened—I’ve always liked that silver plated Thompson, with the jasper grips. The bone stock was a bit ghoulish, admittedly…but I guess I’ll just have to console myself with all that gold and the autogyro.
So much for your Pulitzer though, huh, Senator Rivera? 
Replace the load in John Wilkes Booth’s gun with a spring-loaded flag that reads “Not on my watch, bub.”
Leave a couple of pipes packed with really good kind bud at Independence Hall in Philadelphia right before the constitutional convention got underway.
I’d go back to 1830 and put an elephant tusk on the Galapagos Islands.
1/15/1559
During Elizabeth I’s coronation at Westminster Abbey, I’d be at Westminster Hall setting up a giant video screen and mega-decibel sound system on the south wall. After the coronation party arrived back at the hall for the banquet, midway through the feast I’d announce on the PA, “Folks, this Elizabethan Era is going to knock your socks off”, then blast the Gangnam Style video on continuous loop. Liz would love it, I’m sure.
I’d do the same thing on July 4, 1776 with the Second Continental Congress, at the post-signing ceremony. My guess is that Jefferson would be shaking his hips a bit, but looking around to make sure no one was looking at him; Franklin would be full-tilt Psi-dancing; and Adams would be shaking his finger and scolding Franklin for his heathen behavior.