Try pet sitting. Find friends who are going out of town and offer to take care of their pets while they’re gone. Professional boarding places tend to be very expensive (not to mention impersonal and intimidating for a pet), so your cozy little house or apartment may seem like a very nice alternative. Around the holidays, pet-boarders fill up quickly, meaning you can probably charge more.
Sell things to pedestrians. Entrepreneurial sorts sell things like cold drinks on hot days or hot nuts in winter. This method can be especially lucrative if you set up a spot near a sporting event, park, etc. You’ll need to have a small amount of money to invest in the product, but then you can sell it at a much higher return and make some quick cash.