I think I have a solution to all of this:

For the flip flops:
1. Use masking tape to tape flip flops to your frame. You could tape one to each side of the fork. This would also give your fork a little extra crash protection.

For the water:
2. Put tape over any holes in your frame, but leave one rear-rack-mounting-hole open.
3. Fill your bottom tube and seat tube with cold water and stick a long straw in the hole you left open. You can drink from this as you ride.

And just to make you a little more awesome in general:
4. Get a bento box for anything else you need.

Just trying to help another newbie.