Always keep your answers simple and truthful. "I'm on holiday" is a fine purpose. If asked what you will do on your holiday, "I'm going visit <destination>". "What will you do there?" "I will do <whatever>". "Why such a short trip?" "I don't have a lot of holiday time [always resonates with Americans] and I wanted a break, and I get a lot of airline frequent flyer miles for this." ...

I don't explain anything unless asked to do so. Over-elaboration makes the official mind both curious and unpredictable, and these are not desirable traits in an immigration officer facing you.