Most rashes that are not from internal causes (or broken skin, scratches) are an oil or acid (topical inflammation) and the acid can also be neutralized with baking soda. I used to work in a motorcycle shop filling the batteries. We always kept a couple of boxes of baking soda handy and when we poured the baking soda on a spill of acid, it would sizzle because it was neutralizing it right away. If the spill is too large, the baking soda would just become saturated, so I like the idea of the soap because you can wash it away instead of have the powder all over you.
The banana and watermelon sound good, but I like the coffee best because I can drink the left overs (I like my "spot" of coffee). The weed might be hard to find. I'll find pictures, but it might not be effective on the new ivy.