- First and foremost, a few spares of any medications you may need for conditions you have.
- Gauze pads, tape, and antibiotic ointment for "road rash".
- A few assorted adhesive bandages.
- Hydrocortisone ointment, for insect bites and "butt burn".
- Probably some aspirin or other OTC pain med/NSAID.
- Some money (seriously).
Sunblock and insect repellant don't really count as "first aid" items, but you should have them, generally in a different location, for easy access and in case of leakage. You may also want ear plugs, if you'll be sleeping in tight quarters with other bikers, and snoring borthers you a lot.
Also not in the "first aid" category but worthwhile is "butt butter", especially if you wear standard biking shorts.
In at least 5000 miles of multi-day touring I've only ever needed the road rash stuff, sunblock, insect repellant, hydrocortisone, and aspirin (to help me sleep), plus decongestants that I take for a congenital condition.