It seems as though every week more parents are asking about how to "unschool" the earlier years. This particular collection came out of a request for an "unschooling curriculum" for young kids. I can't help but wonder how much of that is a result of the preponderance of early education and daycare.
You might think in terms like this, to help organize yourself a bit, or think of things to strew:
five senses - it's usually easy to find things to look at, but how can you explore the world with your ears? your nose? your skin? your taste?
big movement - little kids need to do a lot with big muscle groups and it can help to visualize the body and think of fun ways to use different muscles. Arms and legs aren't too hard, but what about chest (breath) muscles? What about ways to twist and bend?
itty bitty - little kids are also fascinated with small movements and very small objects, not just touching and sorting, but picking things up with different kinds of tongs and tweezers. For some kids arts-n-craft stuff fills this niche, but for others puzzles and home-made games, or just collecting small items in nature and taking them apart.
taking things apart is another kind of "category" of things to think about - some kids learn by dis-assembling! And it's often fun. Think about ways to explore the insides of things safely.
states of matter - liquids and solids are fascinating! and for little kids that means direct interaction - often putting their hands right into things, pouring them over or through other things (funnels, whirligigs, strainers), mixing things. Go beyond sand and water and think about rice, beans, flour, dirt, oil, vinegar, mayonnaise... you get the idea! This is an "area of interest" that combines well with some of those other senses - get some cheap spices to add to other mixtures.
seasonal changes are great opportunities to explore new things or old things in new ways - both the natural world, and the human made world of seasonal traditions and holidays. And noticing seasons lets you notice other things - the length of the day, calendars, weather, the sky, animal behavior.
None of these things need to be "presented" in any kind of formal way, although depending on your family's needs you might want to do some pre-planning now and then, so you have what you need handy. A lot of the time it's just a matter of thinking ahead when you're doing your regular shopping - stopping by the cookware section and asking "how could my kid have fun with this?" or looking at a stack of post-it notes and realizing you could turn it into a game of tag, or a climbing challenge. Learn to see the world as a kid - as if everything had Fun Potential! and let your imagination out to play.