If you want to visit the neighboring country of Cambodia, it is possible to finish your Mekong Delta tour in Phnom Penh. If you book one of these tours, the last day of your tour you will take a 3-hour boat trip and arrive to the Vinh Xuong border. You will cross the border, take a 1-hour high speed boat to Lek Luang, where a bus will take you to Phnom Penh.
You can also finish your Mekong Delta tour in Rach Gia. From there, you can take a fast ferry, or a plane, to the paradisiacal island of Phu Quoc.