A male Kenyan Sand Boa or Rough Scaled Sand Boa will get to about 14-18 inches. I have a KSB and plan to get a Rough Scaled soon. Females get around 32 inches so they would need larger. These species however do stay buried quite a bit, but they are pretty cool in my opinion. Kenyan Sand Boas are easier to find and very docile. Rough Scaled Sand Boas have a reputation for being difficult feeders.
A Western Hognose averages 16-25 inches. I'm considering this species as they seem really cool as well. I personally would only put a male in a 10 gallon since they stay smaller.
Rubber Boas average 21 inches for males and 26 inches for females. Either sex would be fine, but these seem to be hard to come by in the US - Canada seems to have more available.
So all of these could live their entire lifetime in a 10 gallon (20lx10wx12h).
Rosy Boas range from 24-42 inches with the males being smaller. I've heard some say a 10-20 gallon would suffice. A 10 gallon might be more for a male. Some say a 10 gallon with these guys might be pushing it.
African Millepedes can get to about 12 inches (but 10 is probably more common). You could keep a pair in a 10 gallon. They like to bury themselves a lot. They eat fruits and vegetables and need a humid environment with soil to dig in. You can handle these guys, but wouldn't want to overdue it. They can let off a noxious smelling substance when frightened that may cause a mild irritant to some - it's meant to taste bad to predators. I handle some regularly (I volunteer at our zoo) and no one has ever witnessed them do this - I'm guessing because they are frequently handled?
Emperor Scorpions (one of the more common in the pet trade) can be kept in a 10 gallon. They are better left unhandled and require a diet of insects. They also don't seem to move a whole lot.
For most tarantulas that cage size would be more than adequate for their lifetime - the don't move around much and are best left unhandled as they are very fragile (if they jump or are dropped on the ground will shatter like glass). They too require a diet of insects.
Guess it all depends on what kind of pet you're looking for. I know the feeling of 'empty tank syndrome'!
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1.0 Ball Python
"Aragorn"
1.0 Kenyan Sand Boa
"Gimli"
0.1 Australian Cattle Dog/Pointer
"Kira"