Making Sloped Yard Fun for 5 Year Old.

Balls! All different sizes, big small medium, and a slip n slide! tell him to lay down and roll down the hill.
With the ball have him invent new games to play.

Get a sheet of plastic.
The kind painters use. Go to your local home improvement store for it.
Lay it down on the grass slope.
With a hose, let the water run on it a tad.
Then, YOU and your son, can SLIDE down it.
FUN!
I had a yard like that when I was a kid.
That is what we would do, and my parents let us, and they did it with us too.
Wear your bathing suits!

Or, roll things down it, and make up races etc.

Jungle gym, swing set, sand box, Slide,
and the things others have said.

I hope he has a wonderful time. Good luck to you and yours.

i might get blasted for this one ;) BUT as crazy as it sounds, a small kid sized hammer (a real one), some nails, and a wood block, is one of my sons favorite outside toys. i think it's awesome to hear him out there hammering. he will find (mom approved) scrap wood, sticks, firewood, and be happy as a clam "building" things. also, bubbles and sidewalk chalk are still big hits at 5.

a selection of old tires layed flat on the ground next to each other as an obstacle course with a balace beam after it. (my kids loved this when they were younger) Also, we always had a tire swing, because all you needed was one tree with a good sturdy branch and a tire to hang there. HOURS of fun on that! Lots of different balls that can be bounced downhill with a big laundry basket near the bottom of the hill to try to "shoot" and get the ball in at the bottom. If the fence is wood, you can use pails of water and paint brushes to "paint" the fence with the water, but it will just dry and go away. No mess or problem. Remote conrtol cars work well on hills, tonka trucks are great on hills, nerf guns and targets are fun in any yard, hill or no hill. Boys love to shoot at things. We always set up targets for our kids to shoot at with their nerf guns. We also like to put up some type of clubhouse or fort in our yard, either the little tykes houses, or the hubby has built forts in our yard at times as well. It all depends on your yard. Kids ALWAYS love to have their own special place to hang out in the yard and have some privacy. We even bought a little tent and some pop up tunnels for the youngest 2 for our yard in place of the fort. That way we could bring it inside when it was rainy or way too hot outside. Have fun!!

Get a nice, sturdy Tonka truck, or something else he can send rolling down the hill - and tie a long piece of twine to it. He can send it down, and pull it up...

Sounds like jungle gym or swings are not going to work on this yard at all, and tobogganing down a slope in a wagon -- wouldn't he end up hitting the fence if the yard slopes right down into the fence? I'd go for balls he can chase (work with him to develop some ball games - like rolling balls of different sizes down the slope to "race" them and see which gets to the bottom first!). Also if he is into "earth mover" or construction type things, get him some diggers to push up and down hill and give him permission (if it's OK with the landlord?) to dig some in specific places, but not all over the yard. Are there any trees? He can use stick and branches and some old blankets and sheets to lean against a tree and make a lean-to style shelter for his own playhouse when the weather's dry.

Wagon. My kids would take our plastic Radio Flyer wagon and go "wagon-bogging" down the hill....just hop in and ride it down the slope. Not sure from your description how safe this would be by himself, but TONS of fun and kept them busy for hours. Good luck!