You can't really counter the damage. Don't be so hard on yourself. You slipped, you know it and you will be better in the future. You may feel some ill effects from the sugar. I get a nasty headache from sugar now that I am living sugar free. I have had some twice since January and I didn't really enjoy it. It's a good learning experience. Next time that ice cream won't be so tempting.

Yes - drink water. Yes - increase your activity level. But this is just to work off the extra calories. To get back into ketosis just takes time, and the right food.