I'd personally only put easy corals in this tank that only need light to survive and not supplemental feedings, such as zoas. Adding inverts that need fed will pollute such a small volume of water instantly, and would require a same day water change.
I'd also try to get an auto top off system so you don't have to worry about adding water daily. There are some pretty cheap and simple ones available.
If you do want to go with inverts, check out sexy shrimp. They are perfect for nanos and are entertaining. I have 5 in my 28 gallon and love them.