Bangkok has many parks, some bigs ones, many quite small.

My favourite is Rama IX Park on Srinakrin Road, I guest the district is Prawet.

Its very large, very well maintained, there are many eating options, a very large lake where you can rent a boat.

A nice day out, especially for a picnic under a shady tree. The weather is quite cool at present (not necessarily typical for this time of year)

Other good ones too, but that's my favourite one.