I would go for water conservation internship. You'll be doing unique work over the summer and will hopefully learn a lot. With any luck, after the summer you'll have something good to write about for your Haas essay or perhaps will even discover a real passion.
Also, one of the the hot fields in business these days is sustainability/clean technology and water is closely related to that. You might get some cool business insights from working in that field.
Unless you're totally into hospitality, I don't see what you would really stand to gain from the receptionist job.
As for part-time jobs at Berkeley, it varies. Most people who work part-time do it as part of work-study, or do undergraduate research.