Coloured ice / ice with things inside - takes a bit of prep.
You can get stripey ice by gradually adding layers with different coloured food colouring. It's a good idea to cool down each layer by putting it in the freezer for a short while before adding it, so that the warmer water doesn't crack the previous one. I also used to have a clear layer between coloured layers so the colours didn't leak into each other.
You can freeze small toys into the ice - again, in stages - first just water, then water plus the toy, then more water on the top.
Sand mousse
Mix play sand with water with lots of washing-up liquid in, and use a hand-help blender to whisk it. The texture is very different.
Large cardboard box big enough to sit in and paint.
Old sheet, tray of paint, make footprints across it.
Make a swamp for toy dinosaurs - soil and greenery in a large tray.
(I had a couple of Lakeland's "oven rack soaking trays" which were great for this sort of activity. They were also very good for clearing lego into later on, especially when models needed to put out of reach of younger visitors.)
Some people have qualms about activities using foodstuffs - but if you don't, or have some out-of-date stuff, then go for it. Jelly, spaghetti (add washing-up liquid to make it slimy), porridge, with added food colouring.
(Actually, as with bread, making jelly on its own is quite fun, and you can add fruit too.)
Washing-up liquid bottle with flour/salt/water mix as for playdough but not cooked. Squirt onto cardboard - when it dries it's sparkly. As ever, multiple bottles with different food colourings is even better.
Papier mache but that has to be spread over a few visits to give the layers a chance to dry.
Bubbles - I remember that one bottle of gelatin in a 2l bottle or washing-up-liquid/water worked well for good strong bubbles, but I can't remember how much wasing-up liquid went in. You can do big bubbles by taking two sticks and fastening a loop of string to the ends. Put the bubble mixture in a food storage box, dip the string in.