We don't have synonyms for plaisance yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are pleasance, laitance and abaisance.
(n) :
Obsolete form of pleasance (“pleasure ground”). [A pleasure ground laid out with shady walks, trees and shrubs, statuary, and ornamental water; a secluded part of a garden.]
(n) :
Obsolete form of pleasance (“pleasure ground”). [A pleasure ground laid out with shady walks, trees and shrubs, statuary, and ornamental water; a secluded part of a garden.]