the truth is, that you can really use basic recipes meant for 8x8 pans without changing a thing, except multiplying the whole thing times (usually) 7 for a whole sheet pan or 3 for a half sheet pan.
Just be sure that if you are making cake batter, that you don't fill the pans more than halfway. Brownies, if they are fudgy can go 2/3 of the way up and shortbread is fantastic, because you can use it as a piecrust as well, baking it off before you top it with chocolate mousse and whipped cream, or keylime style filling or for lemon squares, pumpkin pie filling, etc. Butterscotch brownies are a great one jam packed with walnuts and chocolate chips and all of these can be found in classis cookbooks like the old Betty Crocker or the old Joy of Cooking. When I first starting working in pastry, I used to bake for between 100 to 400 people a day on my own, and used all the recipes of my childhood to get me through. fayefood.com