our yard isn't very big(long skinny city lot) but i try to make it as fun as i can for the kids. this is only our second spring in this house, so we're still working on making it more fun, but here's what we've got so far.
i'm currently in the middle of building a water garden, which i'm hoping will be a great place for the kiddos to explore and play in. it's going to be about 7x12x2(only at the deep end, half will be only about a foot deep) with a little waterfall at the shallow end. i can see them in there a lot with little toys or whatnot playing in the waterfall. but, i guess this is better for older kids. i know when the little one is mobile i'm going to have to watch him like a hawk in the yard near the water. so i guess you'd have to outweigh the cons with the pros with a water garden. most will be planted up pretty well along the edge, and the flat parts i think i will get some planters to put up as a "road block".
we have a pile of stones that dd just loves to build with. she makes the coolest little castles with them. i like building stone sculptures too.
she also has out there all kinds of old baking things like my aluminum muffin cups, pans, spoons, etc. she'll gather her ingredients from the yard and then cooks them up. i need to get on building her a cooking table in a little "kitchen" area.
we have the garden which is always fun to watch all the things grow in. they each have their own shovels and tools and are welcome to plant anything anywhere they want in the yard. that keeps them busy a lot.
the thing that keeps ds's interest the longest is just our hammocks outside though. he really needs the movement and the enclosure sensation is perfect for calming him. i have them strung up with the ends pretty close together so they really encapsulate you when you get in! they are just the simple one person hammock, no spreadders. one is parachute fabric and the other woven cloth.
we also have the pets, which they love to play with outside. the dogs always go out with the kids, and we have a little movable pen for our two guinea pigs that the kids can sit in with them. i'm planning a small water garden/pen for our turtle too.
and of course, they love the swing set, but we didn't build that.
it's just a simple metal one.