I normally avoid the phrase "In a tank that small" as much as I can, but in a tank that small you don't need a skimmer generally. If you plan on going heavy with some gobies and inverts, the Reef Glass can fit in darn near any tank. It's air powered and uses a wooden "airstone". It's priced pretty high for what it is, but the people that have it seem to like it. Outside of that, you'd be stuck with options that sit in your display instead of the back chambers.
If your bioload is low and you're putting low demand corals in your tank I'd stick with water changes. I'd put the money towards an ATO and or a heater controller over a skimmer in this case. The great part about a 5 gallon tank is that you can reset the entire water column with a 5 gallon bucket.
If you plan on going long between water changes though, or are struggling with rising nutrients, a skimmer is definitely worth it. Reef glass if you want to hide it in the back chambers, or something like the small Tunze skimmer or Icecap K1 Nano if you're okay with it in the display.
Wishing you and your tank the best!
