Abstract: Nekrasov correlators are intersection numbers of Chern classes against descendents on the Hilbert scheme of points on a surface. For K3 surfaces these integrals are expected to have quasi-modular behaviour. I will first explain how to explicitly compute the simplest of these correlators using ideas of Ellingsrud-Goettsche-Lehn, and then what can be said about quasi-modularity more generally. The first part is joint work with Cao and Toda.