What to do as Senior?

If I go back I would do the following:

1. Prep for GMAT for the first semester of senior year and try to get a good score while maintaining a good social life / steady GPA. Live a little but also having a good GMAT score basically gives you a good insurance policy in case in few years you completely burn out and need an expensive vacation (i.e. prob me right now after PE). Not sure of prepping deferred MBA as they are more focused on non-finance people but do your research on this (especially HBS/GSB)

2. Prep for on cycle using various resources (WSO/peak frameworks/BIWS, etc.) on your spring semester so it is fresh in your mind when recruiting starts inevitably earlier than last year and you don't have to cram.

3. If you feel pretty burnt out due to last couple of years of recruiting work, be gentle to yourself and relax during your senior year. It is probably one of last years of your life before corporate world where you can actually relax without worrying about actual real world stuff.

Good luck!

I would 1000% in hindsight prepare for on cycle, even if you wont get a MF from a MM bank you can still land somewhere nice if youre educated and it will only get harder after

Recommend enrolling in AARP, great benefits

Honestly, this is probably the last bit of free time you'll have, so I'd suggest 2 things.

1. Learn to code. Even a basic python script can save you a good amount of time and effort, and it will allow you to think in a different way. I did my UG in CS, ended up in tech banking. That CS degree came in handy more times than I can count. You don't need to be some sort of master programmer. Hell, you don't even have to be great. If you can write a basic program to take something off your hands, it's worth the time investment. 
2. Enjoy it. Don't just go to parties and shit, I mean make some deep friendships, and try new shit that you haven't considered before, or something you've been meaning to get around to but just haven't.