We DO put up a regular Christmas tree but anchor it to the wall with fishing line and cuphooks. No breakable ornaments - only fun homemade things from years with the kids - all kitty proof. NO tinsil. We also put the tree up on a small table and this avoids most of the temptation :-) Other "decorating" is minimal and cat friendly.
Aftre seeing Edwina's water fountain, THAT is going to be their gift. I can't find the smaller model so looks like they will get the Deluxe I hope they like it - both insist on running fresh water!!