Title says it all, "What to do with your leftover Champagne" [1]
http://money.cnn.com/2015/01/01/luxury/leftover-champagne-solutions/index.html?hpt=hp_t2

Alternatively try a Sbagliato.
30ml Champagne
30ml Campari
30ml Red Vermouth
1) Rocks glass, stirred with rocks, with an orange slice as you would for a Negroni
or 
2) Stir with ice, strain into a champagne flute.

It's better with some bubbles still in it but doesn't need fresh champagne.

[1]You opened an extra bottle and then didn't finish it? Seriously? ;)