Planning December travel to Eastern Europe....is it too cold? Is there still plenty to do and see?

December is reasonably cold, so you won't be freezing but you'll need to pack accordingly. I think the average is somewhere around 2-4°.

If you're sticking to the cities you'll have plenty to do and see, especially with the winter holidays events, fairs and celebrations.

We spent Christmas and New Year's in Budapest last year. It wasn't so cold that we couldn't get out every day and walk a lot, but there was drizzle quite a bit, and it only snowed, lightly, once. The nice thing about Budapest then is the number of Christmas markets with souvenirs, great food, and hot wine and chestnuts. They even have a giant Ferris Wheel with views out over the city. IIRC it was in the Buda side.

Pjk

Well, Budapest and Slovenia aren't Eastern Europe, they're Central. That said, anything that doesn't move will be there to see - such as Szoborpark, Dohany Utca Synagogue, the Parliament, etc. in Budapest and all the colossi that Ceaucescu built in Bucharest after razing swaths of the city. We did Gdansk and Krakow over New Year's one year, and the weather wasn't terrible. Slovenia and Budapest should be a cakewalk by comparison.

Yeah give it a go it will be worth it. There are going to be a lot of night markets or christmas markets and lights to see in these places. Just remember to pack enough for cold winds or rain.

What is cold to one person may not be cold to another...I'm from Northern California where it gets horribly hot in the summer and "chilly" in the winter. To someone from colder climates, our winter is pathetic.

With that in mind, I was in the region you're heading to (specifically Prague) in early January one year and I found it absolutely unbearable. I was at a conference for students all over the world. The girl from Russia thought it was pleasant...

In the bigger cities, you will have a lot to do indoors like others mentioned. In more outlying areas (say should you decide to visit Bled in Slovenia), most of what to see/do are outdoors.

Pack in layers and hopefully you will do better than me! Have a great trip!