Preheat oven to 350 degrees F (175 degrees C). Grease and flour one 9x13 inch baking pan.
In a large bowl, cream together the butter and 2 cups sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Combine the flour, baking powder and salt; stir into the creamed mixture alternately with the milk. Fold in the rhubarb and strawberries. Spread the batter into the prepared pan.
In a small bowl, mix together the 1/2 cup sugar, 1/2 cup flour and melted butter. Crumble this mixture over the top of the cake.
Bake for 40 to 45 minutes in the preheated oven, until a toothpick inserted into the middle of the cake comes out clean. Cool before cutting.
Interesting Facts
Rhubarb is a vegetable, not a fruit.
Strawberries are the only fruit with seeds on the outside.
Coffee cake is a type of cake usually served as a breakfast item or as a snack.