Milk-based kefir (I have no experience with water-based kefir cultures, but imagine they'd be similar) is fun to make. The grains are easy to handle. It's similar to kombucha in difficulty, but grows faster, and you make smaller batches generally. Like kombucha, it can get quite sour if you leave it too long. I stopped drinking milk so I no longer make it, but remember enjoying the process. I have a Flickr album showing the process here: https://www.flickr.com/photos/davidniergarth/albums/72157604...