Nature Walks! He has a fascination with nature, sticks, flowers, ponds, bushes, fallen leaves. This will be especially fun in the fall. He picks up things and collects them through the walk and we have a spot by the door where he pus them before we come inside. We keep it close to home, whether it's the yard, sidewalk or driveway.
Plant Bucket! This one involves work on your part in the spring but surprisingly will be entertaining for 3 out of 4 seasons. In the spring we planted herbs in a 10 gallon plant bucket, Bear helped maintain the "garden" all spring and summer and now it's empty and makes a fantastic dirt digging excursion. We put it on wheels (set the bucket on a wood board that we screwed 4 wheels to).
Sandbox for Apartment Living! So this is just if you don't want the work of a garden but like the idea of the above. Get a 10 gallon plastic bucket $15 and a couple bags of sand ($? - free if ya live near the beach!) 12x12 wood board (scrap wood is fine) 4 screw on wheels (1.00 each at hardware store) Screw the wheels on the board (corners) and place the bucket on top (you can put a screw through the middle or the weight of the sand should hold it tight. Fill up the bucket 3 inches from the top and throw some digging toys in there. HOURS of fun and since your little one wont be SITTING in the sand no cleaning sand out of cracks (literally) and ruining your lawn. This is great for a deck or porch and would be a great activity to wear him out!
Walks! I mean no stroller. Let your toddler roam around close to you, take a walk with no destination to tucker him out!