In this paper, we focus on spatial modulation (SM) which uses two active transmit (TX) antennas, and we introduce an Enhanced Spatial Modulation (ESM) technique, which conveys information bits not only by the indexes of the active antennas, but also by the constellations transmitted from each of them. The main feature of the proposed technique is that in addition to a primary modulation, it uses a secondary modulation that is obtained through geometric interpolation in the signal space. The secondary modulation has a reduced transmit energy compared to the primary modulation, and this leads to significant performance gains for the same spectral efficiency. We also describe other variants of the proposed ESM scheme in which the primary modulation is partitioned into two or more subsets and the lower-energy subsets are used in a larger number of combinations. We give design examples using four transmit antennas and 16QAM and 64QAM as primary modulations. The proposed techniques are compared to Conventional SM (CSM) and the results indicate that they provide a significant performance gain over CSM in terms of the symbol vector error rate (SVER) performance.