Valence conveyed in simple geometric shapes
I love this kind of research because it shows how there's an affective undercurrent to essentially every part of our lives.  This is formalized in microvalence theory (Lebrecht et al., 2012), which posits that all objects, even ordinary ones, are imbued with valence (e.g., explaining why you prefer one chair over another).  

As far as positive geometric shapes, there is evidence for consistent positive reactions to circular and curvilinear shapes (e.g., Bar &amp; Neta, 2006; see also Larson, Aronoff, &amp; Steuer, 2011).  There may be some obvious top-down mechanisms mediating this effect, such as positive associations with a smile (vs. a frown; Salgado-Montejo, 2015).