I know 4 summer school programs that accept math students between their senior year in college and first year in grad school: the PCMI Summer School, the Princeton Program for Women in Math, EDGE and Nebraska Immerse. All four are fully funded, 3 of them pay a stipend for participation.
If you can afford it, I would highly recommend that you do one of the following:
- finish up undergraduate research (if it's expected to lead to a publication)
- start working with a professor at your graduate institution
- study for prelim or qualifying exams (it's so not fun to do algebra problem sets while your fellow first-year grad students are organizing fun reading groups and travel to cool conferences)