Here are a few links to summer REUs (Research Experiences for Undergraduates).
In addition, fellowships to support Swarthmore students conducting summer research in collaboration with Swarthmore faculty are available from the Division of Natural Sciences and Enginnering. A full description of the fellowships and application process is available from the following link.
Information about the EDGE (Enhancing Diversity in Graduate Education) program, designed to strengthen the ability of women to successfully complete graduate programs in the mathematical sciences, is available on its site listed below.
The American Statistical Association posts a list of internships each year. Many are looking for graduate schools but some are for undergraduates.
The University of Chicago has "Data Science for the Social Good" fellowships:
Summer Institute for Training in Biostatistics