Spontaneous rhymes with:
percutaneous, simultaneous, contemporaneous, extraneous, miscellaneous, percutaneous, simultaneous, acrimonious, aeneas, aerolineas, antonius, bournias, contemporaneous, cyrenius, erroneous, extraneous, felonious, fresenius, harmonious, homogeneous, igneous, ignominious, instantaneous, miscellaneous, morphonios, palonius, parsimonious, percutaneous, phineas, plinius, sanctimonious, simultaneous, thelonius, unceremonious, vilnius
Spontaneous sounds like:
seventeens, spending, submitting, suspending