  Various combinatorial classes such as outerplanar graphs and maps,
series-parallel graphs, substitution-closed classes of permutations and many
more allow bijective encodings by so-called enriched trees, which are rooted
trees with additional structure on the offspring of each node. Using this
universal description we develop sampling procedures that uniformly generate
objects from this classes with a given size $n$ in expected time $O(n)$.The key
ingredient is a representation of enriched trees in terms of decorated
Bienaym\'e--Galton--Watson trees, which allows us to develop a novel
combination of Devroye's efficient sampler for trees (Devroye, 2012) with
Boltzmann sampling techniques. Additionally, we construct expected linear time
samplers for critical Bienaym\'e--Galton--Watson trees having exactly $n$ (out
of $\ge n$ total) nodes with outdegree in some fixed set, enabling uniform
generation for many combinatorial classes such as dissections of polygons.
