Hi smartapple,
I have the same problem with another car which is stored in the garage during the winter. Its battery is so small that I can not start the car after one month.
So I disconnect the battery during these months and connect it to to a charging-control device. Due to the fact you also tried it, maybe it will work if you take the battery out of your smart?
If you battery is quite new I suggest that it also works great if you just disconnect the battery while it is idle. Of course, it's not the best solution...but I think in practice it is ok .

 

Does anyone have any tips for keeping the battery charged while the car is idle?

This is what I use and it is very well engineered. The optional cables let you modify the connections to suit your needs.

Save-A-Battery

Groit's Garage also sells the same the same under their own brand.