Not a big choice, I'm afraid, as this size is too small for most things. Maybe African Violets or small cacti if indoors. Single pansies or violas if outdoors. Too small for any vegetables except maybe single radishes or a juvenile parsley (outdoors)

You could do a microgarden of sorts for indoors if you have sun -- like arugula or cilantro, then harvesting the cotyledons and starting again. It actually looks cute in small pots. Lots of cuttings will take their time and grow slowly but nicely in these pots -- ie gardenias, Arabian jasmine, jade (though that would not be for flowers necessarily), or any succulent, like Flora mentioned.

When they get too big, I often take one out, prune the roots, replace the soil and let it grow again. I don't do that these days but when I had one window ledge in a previous apartment and no space for potting up -- I used to do this almost bimonthly. It's not ideal but it works. :-) Just keep in mind that in summer, if the pot is full of roots, you may be watering a LOT, and I mean like up to 3 times a day haha.