I would recommend splitting the area with one or more retaining walls which can be used to flatten the area and thereby making a good portion usable. You could add in some usable paths and add some plantation too. That will make the area look colorful and make for a good patio.
You will have to level an area by setting some brick/stone in place making a low wall. Then filling that area with soil. From there use some large stones set in concrete (buy the forms from a big box outfit)to make a patio level enough for a chairs and a table.
Decks are always a great idea for sloping yards. You can find many do-it-yourself books at the library for free. Another idea would be to use railroad ties to create tiers.