  Tourism affects not only the tourism industry but also society and
stakeholders such as the environment, local businesses, and residents. Tourism
Recommender Systems (TRS) can be pivotal in promoting sustainable tourism by
guiding travelers toward destinations with minimal negative impact. Our paper
introduces a composite sustainability indicator for a city trip TRS based on
the users' starting point and month of travel. This indicator integrates CO2e
emissions for different transportation modes and analyses destination
popularity and seasonal demand. We quantify city popularity based on user
reviews, points of interest, and search trends from Tripadvisor and Google
Trends data. To calculate a seasonal demand index, we leverage data from
TourMIS and Airbnb. We conducted a user study to explore the fundamental
trade-offs in travel decision-making and determine the weights for our proposed
indicator. Finally, we demonstrate the integration of this indicator into a
TRS, illustrating its ability to deliver sustainable city trip recommendations.
This work lays the foundation for future research by integrating sustainability
measures and contributing to responsible recommendations by TRS.
