Sounds like you’ve got a lot of great experiences! I second what’s been said about research, especially if you can find a faculty member to work under that you get along well with. This can lead to a strong letter of recommendation from your faculty supervisor which is really helpful? Definitely start to think about who you’d like to ask for letters of rec as well, and if you are coming up short start building good relationships with professors (going to office hours, participating in class, etc.)
For GRE, I 100% recommend gregmat.com
His videos are free and awesome. The only prep I did for writing was watching his videos and I got a 5 on that section. Definitely a great resource before you go spending tons of money on GRE prep materials.
Good luck in your senior year and I’m sure you’ll get into a lot of great programs!