Offtopic:
I remember being 4 or so and helping my dad squish the playdough in a plastic bag after it had been cooked. He added cinnamon so it smelled yummy.
On topic:
What about a textures table? One common thing in science museums and things is a table with rice or sand and lots of scoops and things. For out doors you could also add water to the mix by letting the hose pour in as a stream and let him play with blocking off the water flow and digging new channels and stuff.
Songs with hand motions are terrific ways to learn things. (For adults too, incidentally.) Like for colors and shapes you could sing (while holding up the appropriate object) "Red, red, red, red cir-ir-cle, Orange, orange, orange, orange squa-are, yellow, yellow triangle, are the shapes and colors here" You'd both have shapes to hold up and of course you'd hold up the 'right' ones so it wouldn't matter if he was just having fun waving one shape around and didn't want to change.
Now, this last idea is really obvious, but very important, talk to him. As you go about your day, just talk to him about what you're doing and seeing.

On topic:
What about a textures table? One common thing in science museums and things is a table with rice or sand and lots of scoops and things. For out doors you could also add water to the mix by letting the hose pour in as a stream and let him play with blocking off the water flow and digging new channels and stuff.
Songs with hand motions are terrific ways to learn things. (For adults too, incidentally.) Like for colors and shapes you could sing (while holding up the appropriate object) "Red, red, red, red cir-ir-cle, Orange, orange, orange, orange squa-are, yellow, yellow triangle, are the shapes and colors here" You'd both have shapes to hold up and of course you'd hold up the 'right' ones so it wouldn't matter if he was just having fun waving one shape around and didn't want to change.
Now, this last idea is really obvious, but very important, talk to him. As you go about your day, just talk to him about what you're doing and seeing.