visiting Greece need information

We'll be visiting Athens Greece from 12/26 to 01/02. It's our first
time there. Any insight or recommendations on places to see,
restaurants to visit, activities to participate in, (New Years eve??).
Brief phrases to know? What to avoid? Is it possible to take a day
trip to Crete? Any help, tips or tricks will be greatly
appreciated!!!!

Unless you fly-you won't be able to hop over to Crete for the day. It is
about 170km from Athens.

If you want to visit an island for a day you can easily get to the islands
of the Saronic Gulf -Aegina, Poros, Hydra, Spetses etc. But if you are going
to visit the islands, it might be worth spending a few days on one or two
rather than a quick day visit.

If you aim for Hydra you should be able to spend a couple of hours around
the harbour and you will get a quick look at Aegina and Poros on the way,
too.

Have a good time.

PS..I've just noticed that you intend to visit in December. It might not be
so easy to visit the islands at this time of year due to the weather and
reduced ferry schedules.