Sounds like you might have some air in the system. Did you use the
bleeder screw to let air out as you filled it?

If it's been losing coolant without any noticeable leaks, it might be
spitting out of the overflow bottle as you drive. This could be from an
ill-fitting rad cap (if it doesn't hold the coolant at the correct
pressure, it lowers the boiling point which in turn forces coolant out
as it vaporizes), get a new OEM cap at your Honda dealer, about $15-20.
If this doesn't work, it could be a leaking head gasket, either
allowing combustion gasses to enter the cooling system or allowing
coolant to leak elsewhere in the engine.

I know the coolant temp sensor doesn't work correctly when it's not
submerged in fluid, so this might be sending false readings, telling
the computer the cars still cold.

I recently flushed the rad in my 1990 Honda Civic wagon and replaced
all the coolant as well. I put a pellet of leak stop in there as well
as I seemed to be losing a small amount of coolant over time but could
not determine where the leak was from.

After flushing the rad, I've noticed
i) the heater on my car is not as powerful as it used to be
ii) the idle on the car, even after the engine has warmed up is around
1100-1200rpm.

Any ideas what may be causing this and how to remedy either of the
above problems?

Thanks again for all of your help. My previous problem with the gas
tank was that it was corroded in the neck and the expansion of the cold
fuel managed to break the tank. I've since replaced the tank and no
longer have a gas leak.