We wanted to reroute more gutters to the rain garden but I didn't want to do anything permanent until we'd really tested whether it could handle so much more water. My first thought was a racquetball over the downspout hole (I don't know) but Greg didn't have one, despite the fact that he owns every piece of sporting equipment ever.
But a measuring cup worked. Don't laugh.
Believe it or not, this is effectively blocking that downspout and the water is now dumping into the rain garden (which is now filling a lot faster). I can watch it during heavy rain and see if it's in danger of overflowing. If the extra rain overflows or overwhelms the rain garden, I can just yank the measuring cup out of the gutter and take the pressure off.
And if it continues to work we can have that downspout removed professionally. And I own three measuring cup sets so I should survive without this one. Everyone wins! Now stop laughing.