At Golden Triangle Park, you will find a triangle with the 3 materialized countries (see the picture), an arch and a kind of large stylized boat installation with a Buddhist sanctuary with a sitting Buddha. It is also possible to take a boat tour from there to cross the river and go to a market in Laos (market famous for its counterfeits) or on the Laotian island of Don Sao. This is not a point border crossing point between the two countries, so you must leave your passport before getting on the boat which is normally forbidden. So, I do not advise you these boat tours. Many consider the Golden Triangle to be a must-see of any visit to Chiang Rai, but if being so close to two other countries can be attractive, some travelers are a little disappointed but it remains a curiosity. If you come here, take some time to visit Chiang Saen surroundings, there are some interesting temples like Wat Phra That Pha Ngao in Chiang Saen city, Wat Phra That Doi Pu Khao and some others in the area.
To go visit Golden Triangle, you can also book one of the tours above.