New here and somewhere I must start trying to give my 2 cents.
Where capelo said is really the best place for me too in Asuncion, then if your closer to Lambare or city centre going south down the Gral. Santos there's a couple of places too just before reaching Cero Lambare (Where I mostly go) which isn't to bad either.
Then if you have the means and time there a place just outside of Caacupe, which is called the Centro Expo Flora, haven't stopped there in years though so I don't know how they are going on. You'll find it close to that "Cruz de Peregrino" on the lane that comes back to Asu, and of course theres Caacupe it self which has lots of families making a living out of gardening planets.