What are some of the most tried, tested and proven co-marketing activities to propose to tech pârtners for the objective of lead gen?

Co-Hosted Webinars have worked really well for us.

This is a much debated topic, which varies by industry. But what we have seen in our SMB B2B market is that collaboration at conferences/expos/trade shows have the biggest short term impact for lead gen. The next best would be cross selling blitz calling between the two partner's customer bases. Make sure to have a short clean partnership value prop, and have both partner and company sales teams on the calls together prospecting. Webinars are not very impactful, for anyone, customer, partner, company.

One of the easiest ways to co-market with a tech partner is by jointly creating a single asset (PDF, video etc) and having that asset live behind a gated form that has a double opt-in to receive communications from you and the tech partner (aka new leads!).