The title pretty much says it all. I want to see how much rep is available today. If I click on the little envelope, it shows all of my rep number for today, but it does not aggregate and it includes accepted answers. I just want to see how much rep I can earn today from non-accepted answers, before hitting the rep cap.
There's no direct way to see how much more until you hit the cap, but you can calculate it. Take your current score (which I assume you already know how to find) and subtract accepted answers and bounties awarded. The amount left is 200 minus the score you just calculated.
To see all reputation activity for a certain time period, use the "reputation" tab in your recent activity pages. If you prefer a more text-y format for consuming the same data, you can use the reputation audit page instead.
Also, I should point out that the reputation "cap" only applies to question and answer votes. You can earn more than 200 reputation in a day by having answers accepted or by being awarded a bounty. The calculation in the first paragraph tells you how much more reputation you can earn from question and answer votes only.