We have a tree from my husbands Uni days (or as I call it the Charlie Brown christmas tree), which none of my cats have really bothered with. Last year my foster kitty Harley managed to knock the whole thing over using her very bushy tail - she didn't actually mean too.
This is her first year with the in laws so I'm sure I'll get stories of what she's done, this is also our Gizmo's first Christmas.
I have however banned the cats from the living room (due to the TV vs Lirael problem) so our tree will be safe, the in laws tree though.... is fair game!