To determine which activity has the greater potential for nurturing creative and critical thinking, let's evaluate each option:

1. Preparing a research summary: This activity involves summarizing existing research, which primarily focuses on comprehension and analytical skills rather than creative and critical thinking. The main task is understanding and condensing information, not necessarily creating new ideas.
2. Presenting a seminar paper: While this activity requires understanding and explaining research, it is more about communication skills. There might be some room for critical thinking in terms of how the content is presented, but it doesn't inherently demand the generation of novel ideas or innovative thinking.
3. Participation in a research conference: This activity encourages participants to engage with diverse perspectives, discuss and debate new research findings, and form connections with other researchers. Such engagement fosters both critical evaluation of new research and creative thinking in how these findings can be applied or expanded upon. Thus, it nurtures both creative and critical thinking.
4. Participation in a workshop: Workshops are often hands-on and can be creative, but they tend to focus more on practicing established techniques rather than fostering high-level critical analysis or creativity.

Based on the analysis, participation in a research conference offers the greatest potential for nurturing creative and critical thinking. It provides a platform to explore new ideas, question existing knowledge, and collaboratively advance understanding in a particular field.