In this article, we propose a region-based self-triggered control (STC) scheme for nonlinear systems. The state space is partitioned into a finite number of regions, each of which is associated to a uniform interevent time. The controller, at each sampling time instant, checks to which region does the current state belong, and correspondingly decides the next sampling time instant. To derive the regions along with their corresponding interevent times, we use approximations of isochronous manifolds, a notion first introduced in Anta and Tabuada (2012). This article addresses some theoretical issues of Anta and Tabuada (2012) and proposes an effective computational approach that generates approximations of isochronous manifolds, thus enabling the region-based STC scheme. The efficiency of both our theoretical results and the proposed algorithm is demonstrated through simulation examples.