Will the AirTag work with an iPhone 6s Plus?

If you actually have a 6S plus and not a 6 plus, yes, you should be able to use AirTags with your phone. From the article linked to above:

An iPhone, iPad, or iPod touch with iOS 14.5 or iPadOS 14.5 or later and two-factor authentication turned on. 

Keep in mind that you cannot share AirTags with another Apple ID, so, if you are traveling with someone, they will not have access to the location data. For some people, this is a deal breaker. If it is for you, you may want to look at other tracking tags. But, if it's not, you should be fine.

Have a great trip!