We study the extremal properties of a stochastic process x
t
defined by the Langevin equation
x
̇
t
=
2
D
t
ξ
t
, in which ξ
t
is a Gaussian white noise with zero mean and D
t
is a stochastic ‘diffusivity’, defined as a functional of independent Brownian motion B
t
. We focus on three choices for the random diffusivity D
t
: cut-off Brownian motion, D
t
∼ Θ(B
t
), where Θ(x) is the Heaviside step function; geometric Brownian motion, D
t
∼ exp(−B
t
); and a superdiffusive process based on squared Brownian motion,
D
t
∼
B
t
2
. For these cases we derive exact expressions for the probability density functions of the maximal positive displacement and of the range of the process x
t
on the time interval t ∈ (0, T). We discuss the asymptotic behaviours of the associated probability density functions, compare these against the behaviour of the corresponding properties of standard Brownian motion with constant diffusivity (D
t
= D
0) and also analyse the typical behaviour of the probability density functions which is observed for a majority of realisations of the stochastic diffusivity process.