One of the best fertilizers for a garden of any kind (or lawn!) is well composted manure from chickens. If you can't or don't want chickens and don't know anyone who has them, you can buy it, but here's a frugal way to get the same benefit.
Chickens and birds' manure is basically the same, so if you have birds around (and who doesn't?) you can encourage them to come and stay in a certain areas of your yard by providing food and water. It won't be long until the ground under these provisions will be splattered with bird droppings. That's gold for your garden! Rake or sweep up the area and put the droppings, along with soil, leaves and so on, into a compost bin or a marked hole in the ground and let it compost. If you put it in the ground and leave it, it will turn into rich compost within a month or so. A compost bin, regularly turned, will take less time.
Of course, this will make only a tiny bit of compost, since you will only have a tiny bit of manure, but if you want to make more at one time, scoop up the droppings and store them in a dry place until you have enough. You might be surprised at how quickly it adds up!
While most cities don't allow small livestock farming within their limits, it's all in the definition. "Small livestock" can be pets - or at least be pets in the eyes of the city and the neighbors. That includes unusual breeds of chickens or rabbits of almost any kind.
Be sure to know your city rules thoroughly, then proceed at your own risk. If you have pets of this kind, you will have to be careful of their housing so that you are not seen as abusing them. If you have nosy neighbors, a privacy fence is a necessity.
If your pet rabbit has a litter, you will have to decide what to do with the little ones. You can raise them and take them to someone who will butcher them for you, since you probably won't be allowed to butcher them yourself in the city limits. You can sell them as pets or anything else and make a few dollars.
I have raised rabbits, but I am not an expert, so I will hand you off to this site if you decide to raise them: Naturally Feeding Rabbits This method can save you a bundle of money and make for a happy and healthy rabbitry. Nature triumphs science any day.
You probably won't be allowed roosters in the city limits, so clutches of baby chicks won't be a problem. See more about keeping chickens here: Chickens in the City
It takes common sense and courage, but you may very well be able to add an animal or two to your backyard homestead.
A few flowers (some wild) helps make it a better. It's not up to city standards of beauty, but it works for me. The more natural look I can get into this place, the more comfortable I am in it!
Here is another view of the same area. The pots with red salvia are cheap plastic, spray painted. There is a purple button flower that you can't see and some wild salsify. Later, there should be tiny mallow flowers and maybe something else that hasn't peeped up yet.
The area isn't finished yet and may never be! I am thinking of putting in a couple of gooseberry bushes by the fence. What do you think?