I think you're approaching your bschool application like a checklist:
College degree: check
GMAT: in progress
Job: check
Volunteer work: nothing
I personally don't think this is the right approach. Business schools nowadays want to see the entire person. Sure, you can craft an essay, take a standardized exam, hold down a job, but who are you besides that? Volunteering for a week in Africa isn't going to do anything for your application, unless it means something to you and can add to your overall story. Do you have family in Africa whose community you want to help? Or are you just finding a way to volunteer in Africa because you think it will sound impressive to an adcom? It won't.
Yes, you should be involved in your community in some way. And yes, everyone is busy and certainly we all might have "better" things to do than [insert generic volunteer opportunity here]. However, you should be a part of your community in a way that will be impactful to you and your community -- whether that's something more "traditional" like helping out in a senior center or a food kitchen, or something a little more "off the beaten path", perhaps you work with an organization bringing music to children because you've played the cello for 10 years or you help to bring local produce to the community because you worked in a grocery store in high school or you grew up on a farm.
Bottom line: Do something that matters to you, that you would be proud to say you're apart of. Don't volunteer for a week here or there just because you think you have to.
For more on this topic, visit the
Volunteer experience hypothetical thread. It could give you more to think about.
*All situations listed above are hypothetical.