Ice play. Freeze a few small toys in a tub, maybe add some food colouring, pebbles, whatever. I used a cake ring mold that worked well. When frozen, let them at it with sticks, spoons, salt shaker, jug of warm water, etc and let them excavate their toys. If you do it in a plastic tub or basin, the toys can be washed afterwards for double fun.
I also liked shaving foam on a tray with ice cubes (some coloured is nice).
If you do bubbles, try making a bubble snake: cut the bottom off a little juice bottle, cover with an odd sock and dip the flat sock end in the bubble mix. Blow slowly through the drinking end. Bubbly snake will grow.
Treasure hunt, try to find as many different leaves, stones, twigs as you can then glue them to a cereal box for a collage type thing. Works well outside as you can hose them off afterwards!
Have fun.
Good idea with the toy car slide niffernaffer, I'm going to do that today.