$2 day plan + $1 day data plan

I currently have a pay as you go plan. I want to use data when I go on a trip, if I have my phone turned on but do not use it, is there a charge? No. You are only charged if you use the phone. what if I receive a text or phone call but do not reply is there a charge? Yes. If you receive a call or text, you will get the $2 daily charge So I understand this plan would cost $3 a day? is this correct? Not necessarily. You are only charged the daily $2 charge if you use the phone for calls or texts. It is possible to purchase the $1 data add on and not get charged the daily $2 charge as long as you only use data and do not call or text. 

Added:   The $1 data add on expires in 24 hours, used or not.

since this is a current plan, you might consider a monthly plan with a data package for the month.  

https://www.att.com/prepaid/plans.html