The researchers found that sixth-grade students at Traimit Pattana Suksa School had limited problem-solving and mathematical reasoning skills, which was attributed to the way their learning activities were organized by their teachers. The traditional approach did not allow students the freedom to think and practice solving a variety of problems in unconventional ways that mirror everyday life. To address this, the researchers applied an open approach to organizing activities and developed learning activities that fostered problem-solving and mathematical reasoning skills of the students. The goal of the study was to achieve an average score of not less than 70% using action research based on the concepts of Kemmis and McTaggart. Data were collected using a Sub-test at the end of the operating spiral, Mathematical Problem-Solving Ability Test, Math Reasoning Ability Test, and student behavior observation form. The data were analyzed using descriptive statistics, including percentage, mean, and standard deviation. The findings showed that the open approach to organizing activities can effectively develop the mathematical problem-solving and reasoning capabilities of students. The students were able to create work pieces, explain different types of 3D geometric shapes, show how to find the volume of a rectangular shape from given problem situations, and provide reasons to verify their ideas. According to the test results, 13 students (81.25% of the total number of students) had the ability to solve mathematics problems at 70% or higher, and 15 students (93.75% of the total number of students) had mathematical reasoning ability that met the criteria of 70% or higher.