I would look into the semi tropical or temperate water fish, rather than tropical, if the temperature will get into the lower 60s. There are a few tropical fish that will tolerate temperatures into the upper 60s, but not much lower.
The high end in the summer is also a worry. If a fish can handle the upper 60s it is rare that it will also tolerate the 80s.
Here are a few cold water ideas:
Platy (a single male or maybe 2, but this is not good. They are social fish)
Rosy Barb (a single fish, but, like Platies, these are social, and a larger tank and more fish is better) Other Barbs that you might see listed as cool water fish, similar comment. Not enough room in a 5 gallon, and some Barbs nip plants. You sure do not want that with a Lace Plant!
Darters and Gobies- look into native fish, and check the laws about catching them. In some areas non-game fish are all counted as 'bait' and you can catch and keep them. In other areas this is not legal. Most of these need more water movement, too. High oxygen water.
Hillstream Loaches are NOT a good idea, though you may see they are suggested for cool water. They really need more water movement for higher oxygen.
OK, here is one for you to research:
Florida Flag Fish. A single, perhaps male (more color).
MAYBE: other Killiefish??? I am not really into these, so you will have to do your own research.
There are quite a few Cories that will be fine into the lowest 70s, and a few as low as the mid 60s. Research at Planet Catfish. They are social, so the only way to have a good group in that small a tank is to go with the smallest species. There are several, but I do not think they are tolerant of the colder water. There is a really good search engine over there, but of course it just covers cats.
Sorry, it looks like it is coming down to Mosquito Fish. That is just too great a temperature range for most fish.