The £350,000 event was held at West Midlands Safari Park on Saturday and featured a family fun day in the park's giant private marquee Tree Tops, and an evening event in the recently rebuilt Spring Grove House.
The event featured displays telling the agency's history and updating guests on the progress made through its Thirty 30s scheme.
Constanti said: "Thomas Cook have enjoyed a relationship with the DRP Group for 27 years and it is events like this which remind us all exactly why we choose to work with them. The whole team pulled out all the stops to put on an event of the highest standard."
DRP Group managing director Dale Parmenter said: "Seeing the culmination of months of work, some of our 30th projects and the exceptional level of creativity demonstrated here today reinforces my belief that the DRP Group will continue to go from strength to strength in the next 30 years as it has done in the last 30."
See photos of DRP Group's 30th anniversary party.