The suggested stations show up when you're viewing various pages for different cities around the globe. It doesn't look like they were created custom for the TripAdvisor partnership but rather are existing stations that have been curated and linked to various cities. They're built on the same Songza technology that Google has been using for its stations since late 2014. The connection to travel may be slightly dubious here, but Play Music's activity- and mood-based stations remain one of the service's best features, so exposing it to more potential users is a reasonably good idea.
Ultimately, the goal is to push users into giving the free trial a shot and eventually converting a subset of those into full paying members, but how successful it'll be remains to be seen. Despite it being a somewhat odd and obscure partnership, two free months of a strong streaming music service is hard to pass up. If you haven't tried Google Play Music, you can get this promo starting today through the TripAdvisor Android app.