Synonyms for Impromptu (same or very similar meaning)
WordNet sense 1 (with little or no preparation or forethought):
ad-lib, extemporaneous, extempore, off-the-cuff, offhand, offhanded, unrehearsed, extemporary, offhandWordNet sense 2 (without advance preparation):
spontaneously, ad lib, ad libitumFrom the ODE community, based on WordNetadd/edit