Any International plans besides IDP?

Not or your current plan.  Your only other option is pay per use, which is pricey. 

IDP charges you $10 plus tax to use your domestic plan on any day you send a text (incoming is free) make or receive a call or use data.  Max per bill cycle is 10 days charges. 

Thank you for your reply!

BTW, if I did not apply for IDP, would the pay-per-use option be default when I go aboard? Or do I need to apply for roaming in order to go pay-per-use?

(As I won't use my phone very often when I travel)

BTW, if I did not apply for IDP,

It's not an application, it's merely adding it to your plan as a feature or an option. You're not purchasing anything unless you use service while connected to a foreign service provider. 

would the pay-per-use option be default when I go aboard?

Yes. Pay per use is a very expensive option and you could exceed the $10 per day limits that International day pass provides very easily.   Data costs $2 per megabyte, not GB, per megabyte. Text messages sent cost $0.50 multimedia messages sent cost $1.30. Phone calls are anywhere between $2 and $4 per minute depending on which country you are in. 

Or do I need to apply for roaming in order to go pay-per-use?

There is no application.   But you may block pay per use roaming by 'un-checking' the $0 travel pay per use feature on your line in your myATT account. 

(As I won't use my phone very often when I travel)

Data roaming is usually off by default in settings on phones, but check it. You could leave data roaming off, not answer and dismiss calls if your phone rings. (you still may incur a 1 minute charge if the phone rings). This would at least limit your charges while outside of the United States.