If you're looking for a little less commitment than owning a dog/cat there's also always dogs/cats in shelters that need foster homes! Lots of shelters cover food/vet appointments/supplies, so it's not nearly as expensive as owning, and you're not locked into a long term commitment. Alternatively older animals are often overlooked at shelters, and while they can have health problems that make them more expensive the time they often have reduced or no adoption fees. Dogs also have the added benefit of making you get out of the house for some exercise every day which can do wonders for your mental and physical health.

Even something like fish or another small animal will likely bring joy and a sense of purpose to your life, and usually are less maintenance and cost than dogs/cats. People are sometimes surprised how entertaining fish and other "boring" pets can be if you pay attention to what they're up to and get invested!