Here's a good idea: Make donations, especially during holiday season. It raises your profile and creates good feelings about your company. Plus, it's the right thing to do. Here's another good idea: Find ways to get overmessaged customers to engage with your brand by posting content or comments to your site or social media pages.

It turns out the two are a pretty good match. And this holiday season, some smart businesses are combining both into one unstoppable force for customer engagement.

It is genius, because jaded customers often won't bother to post a comment or respond to a question--unless you give them a good reason. Making the world a better place without having to spend their own hard-earned cash is a really good reason.

Perhaps the granddaddy of this concept is the nonprofit FreeRice, which invites visitors to play a simple definition word game or math game and donates 10 grains of rice via the United Nations for every correct answer--educating visitors and feeding the hungry at the same time. (The donations are paid by advertising that runs on the site.)

Building on that concept, here's what three startups are doing:

1. Post a review. Send a needy kid a gift.

Inc. 5000 company TechnologyAdvice provides unbiased advice to help companies choose the right business technology for their needs. The company collects user reviews as part of its stringent product research. So until December 17, for every product review users post, the company will donate $10 toward holiday presents for troubled children. The donations go to Youth Villages, a nonprofit that helps troubled children and teens, and provides them presents at holiday time. The kids get the presents they're longing for, the company gets more reviews and increased user engagement, and the users get the satisfaction of helping needy kids. Everyone wins. In September, TechnologyAdvice gave away a $1,000 Amazon gift card to encourage users to post reviews. That worked--but response to this program has been much more powerful.

2. Tweet your kid's favorite toy. Help a child with brain damage.

The e-commerce site Flubit is going head-to-head against Amazon, promising to beat its price on any individual item (in the U.K. only, for now). This holiday season, Flubit is upping the ante. The company's inviting parents to tweet their children's favorite toy to @flubitweet or post it on the company's Facebook page, using the hashtag #flubittoys. For every tweet or post, the company will donate 1 pound to The Children's Trust, a nonprofit that assists children with brain damage. To sweeten the deal, Flubit will also pick one lucky tweeter or poster to receive a sackful of presents.

3. Download an app. Provide needed pregnancy care.

The pregnancy and childbirth site The Bump recently added charities to its gift registry, an easy way to let expectant mothers ask friends for donations rather than gifts. To launch the new offering, The Bump ran a promotion named "Bump It Forward." Each time someone downloaded one of its free pregnancy or baby apps for Android or iTunes, the company made an undisclosed donation to Every Mother Counts. That's a nonprofit that provides prenatal and childbirth care to women who can't afford it all over the world, including in the United States.

With almost a month till Christmas, it may not be too late to put together a customer-engagement/donation combo of your own. How would you like your customers to engage with your brand? And what charity would you like to support?