To help others, you need to take care of yourself first. If a membership to a gym or yoga studio will encourage you to exercise, maybe you should consider investing in one.
Remember, your mental health needs to be taken care of, too. You can spend your tax refund on a massage to relax or perhaps even a therapy session if it's really necessary.
You should also schedule and invest in health checkups if you haven't been able to afford it. If you have good coverage, your insurance should be able to foot a large part of the bill, but be sure to check with them beforehand. No health insurance? Use your tax refund and try to save up for health insurance.