The notion that it is gardening only if you grow plants outside seems disingenuous to me. After all, if your plants are healthy they must be getting from you what they need to thrive. Right? That is gardening, right?
If you only have space indoors for plants, African violets make very proper flowering houseplants. Place several pots of different cultivars or colors together on a table and voila, you have a garden.
In Milwaukee, mine flowered right around Thanksgiving. The collection made quite a show against the back drop of snow covered windows.
Consider miniature African violets for open terrarium planters, only allowing water to travel down the sides of the glass. I have seen gardeners grow tiny ones in old tea cups, not a permanent pot but pretty enough to enjoy for a time.
Some simple tips for growing violets indoors; do not let water drip onto leaves, no misting, and use a humus rich potting soil mix or one labeled for African violets.
Older African violet plants will develop multiple crowns, suckers or in some way need rejuvenating. They can be propagated by leaf or plant stem to create a larger collection or to share with others.
Want another challenge? Try Gloxinias, a relative of African violets. My dad loved these plants but I could never grow them successfully for long. Perhaps this challenge is just what you need to stay gardening.