1. Happy Holiday(s)
When in doubt, go back to the classic, all-encompassing "Happy Holidays." To make it more specific, use the singular Happy Holiday, allowing the receiver to mentally choose which Holiday you are referring to.
2. Season's Greetings
This is the easiest and also transferrable to any time whenever. Say it for Valentine's Day, Thanksgiving, Flag Day. If you fully commit, you will never have to think of another greeting ever again.
3. 'Tis the Season
'Tis is such a good word. In fact, we should start using 'tis instead of it's. 'Tis much more mellifluous.
4. Merry etc.
People often say Merry Christmas, Hannukah, etc. So just cut straight to the point and say Merry etc. This is far more inclusive and time-saving.
5. Have A Good That Thing You Celebrate
This is especially helpful for your friends whose religious/cultural beliefs differ from yours.
6. Happy *mumbles..*
Whenever greeting someone, trail off after the initial "Happy." If anyone questions you or attempts to clarify insist on talking about the weather.
A good "Congratulations" can double for those who are celebrating an event that led to a universally recognized holiday being formed and those winter graduates. Con-GRAD-ulations in your studies and/or major religious events! Either way, have a great new year!
8. Yaayyy Celebrations!
I love this time of year because so many things are happening. Parties. Color Coordination. Giving. Receiving. 'Tis great, let's celebrate and buy things!!
9. Happy Pre-Halloween
It's only another 11 months away!
10. Can't Believe the Year's Almost Over, huh?
Greet your friends and simultaneously bond over the inability of the human mind to grasp simple theoretical concepts in practice.
11. Happy New Year!
I suppose "Happy New Year" would work too, but that's a bit cliche. I mean, people say it EVERY year.
12. Hello :)
Just checked, apparently hello still works. Use a "What's Up?" at your own risk, though.