I'm lucky enough to have received acceptances, but I was looking at a shutout a week and a half ago. I had a couple plans in place:
IRT for application help and fee waivers (mentioned by @WildeThing) https://www.andover.edu/about/outreach/irt
Revise writing sample--I submitted my senior thesis, and (sloppily and hastily) edited it to meet the page count. Submit to a conference (as a couple mentors have mentioned this would be a good conference paper.)
Have mentors read Statements of Purpose and ask for feedback--I procrastinated on these and only wrote them after my letter writers had submitted their rec letters so I felt too behind to show it to them for feedback at that point.*
Read material from POIs to customize SoP more. There was a good post discussing how people identified "fit" recently that you might want to check out! (Note: If you don't otherwise have access to academic resources, you might look into your nearest public university's library.)
I considered studying for the GRE (quant section) and the subject test. There was a recent post about how the GRE quant section didn't matter at all, so I think I would recommend against that after all. Not sure to what extent the subject test matters, but I would also wait a bit to see if any schools will change whether or not it is required.
*Edit: I would also try to supply my LOR writers with my application materials well ahead of time. I met with each of them to talk about what schools to apply to, but I think it would have been helpful for me to supply them with at least my CV and SoP.