constant, stable, stationary, steady, unchanging, unvarying

fast, fixed, hard-and-fast, immutable, inflexible, invariable, unalterable, unchangeableestablished, set, settledceaseless, continuing, durable, enduring, lasting, permanent

on an even keel(also on even keel)

capricious, changeful, changing, fickle, fluctuating, fluid, inconstant, mercurial, skittish, uncertain, unpredictable, unsettled, unstable, unsteady, varying, volatile

adaptable, alterable, changeable, flexible, mutable, variableephemeral, evanescent, fleeting, momentary, transient, transitorykaleidoscopic, phantasmagoric(or phantasmagorical)