Duplo. Put together a basic structure like a few trees (jungle) and get out the other toys (teddies, whatever) to "play" in the jungle/have a picnic/whatever.

Playdoh + few other random items they have not playdohed with before (eg, spoons, plastic toys, etc) - roll the dough out & show them how to press the things in to make a "picture".

I second anything else with water play or bubbles - you can set up in the kitchen in a tub on a table if it's too hot to go outside.

Print out some Christmas pictures & do coloring & cutting. Stick then to a small paper plate. Loop some ribbon through the top, and voila - Christmas deco!