Creating a junction object between Campaign and Opportunity is the best way to track which marketing activities influenced a customer to make a purchase. This junction object will allow you to track the influence of multiple Campaigns on a single Opportunity, which can be used to gain insights into which Campaigns are most effective.

Setting project milestones and establishing key performance indicators (KPIs) is crucial for ensuring successful implementation and adoption of a Sales Cloud project. This approach provides a clear roadmap for the project, helps in tracking progress, ensures accountability, and measures success against defined objectives. Milestones and KPIs guide the project team and stakeholders through the implementation process, ensuring that each phase is completed as planned and that the project's goals are met.
Reference:
Project Management for Salesforce Implementations
Defining and Measuring KPIs