There are many reasons to develop your own cottage garden space. I would even argue you don't even need a garden and that the same rules apply if you wanted a collection of potted plants for your patio or balcony. It's about creating a space of beauty. You go there to relax, work, meditate, interact with and invite nature into your garden.
In a way, a cottage garden is a living entity that's always evolving and transforming. It can be developed over years or even decades. It is not formal, and it can adjust to whatever space you have to work with. These gardens can take a couple years to get established, and you will find key perennials returning year after year to put on a show. Of course, some plants will eventually run their course and need to be replaced by newer plants.
It's good to think about using heirloom flowers in the cottage garden. This will allow your plants to re-seed every year so they don't need to be purchased and planted all over again. But like I said, there are no rules. So don't be afraid to try a hybrid flower, but you might get something very different from those seeds the next year. The heirloom plant will just keep giving you a copy of the same plant every consecutive year.
I generally think of mostly flowers and beauty when I think of a cottage garden. But there's nothing wrong with planting ornamental herbs, fruits and vegetables in there as well. If you can check multiple boxes with flowering plants that also produce...well, produce, even better.
If you look at people in the Blue Zones, those areas on the planet where people live the longest and healthiest, they are often filled with older adults gardening. All that bending, lifting, planting, shoveling, and pulling weeds is great low impact physical exercise. This not only benefits the human body, it clears the mind in something I would describe as similar to meditation. There is a sense of accomplishment when you tidy up the garden and see the fruits of your labors (all those blooming flowers).
One benefit of the cottage garden is that it can be an oasis for pollinators like the bees. You would be surprised at all the fascinating critters a garden of flowers attracts, from hummingbirds to giant butterflies to gorgeous birds. I have always had a bird bath in my gardens, it's pretty much a requirement for me. It's amazing how excited I can get when some sort of finch or colorful bird shows up to take advantage of it. While not a requirement by any means, it's fun to add a point of interest to your garden with a bird bath, arbor, picket fence, bench, swing, shed or classic urn. Even a distinguished looking stone can be highly decorative.
It's always good to have a wide variety of flowers in the space so that there is always something in bloom, preferably multiple flowers. I recommend flowers that have longer blooming periods mixed in with those that have shorter displays. While it's nice to have iris and peonies, they look spectacular for a few weeks, but then they need another year to recharge.
There are a seemingly endless list of plants you could grow in your cottage garden. Though often the standard clichés you hear about them performing so well, it's hard to overlook them. Roses, dahlias, hollyhocks, zinnias, sunflowers, delphinium, foxglove, snapdragon, lavender, bee balm, cosmos, poppies, daisy, sweet pea, clematis, peony, daisy, dianthus, hardy geraniums, phlox, lupins, honeysuckle, and many more make good choices. Just be careful if anything could be invasive, which might include flowers such as morning glory, daylily, bamboo, French hollyhock, and mint. While many ornamental flowers are actually toxic when consumed, it's good to make sure you're not adding poisonous plants to the garden that may be consumed by children or pets.
If you live in a more extreme climate, say hotter or colder, you will want to seek out plants that tolerate those conditions or are native to the area. That can also be fun, so don't be put off by those limitations.
Be patient with the garden and consider it an ongoing, living entity and passion project that will take some time to get established. But will deliver so much in return.