While I think calendars and similar are a better use of space than leaving it blank I can't think of a single card I've ever kept simply because it had one on the back I think that space is best utilized toward marketing - Lisa's list on the back is perfect, as would be a tagline, a motto, a quote - something to make an impact on the receiving party and impress upon them a key feature of your business or even just educate them altogether as to what a VA is (A spunky definition of a VA would be good back there too).
It's like a mini elevator speech

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