In a presentation, "What we have done" is often titled Accomplishments. Sometimes people use Results, but strictly speaking, results are the consequences of what was done, while accomplishments is understood to more broadly include the work that went into achieving the results. In a context of a long list of things to get done toward the final goal, such as the many, many steps that go into building an office building, the list might simply be titled Completed. The choice among these words is really about what kind of value judgement you are making.
- You worked hard and generated valuable results that you deserve praise for: Accomplishments. Typical in a business setting where you did some things that had a positive impact on the business.
- You did some research or experiments to figure something out and indeed, figured it out ("What we found out"): Results or Conclusions. Scientific papers have both sections, with Results being the most objective measurements possible and Conclusions being the interpretation of the results into some more meaningful concept. We gave 100 people a pill a day for a year. At the end of the year, 50 of them lost over 10% or more of their starting weight and 20 of them died. (Those are results.) The conclusion is that the pill is a powerful weight loss promoter but the health risks outweigh the benefits in all but the most morbidly obese patients.
- You are planning a wedding and got invitations picked out, printed, and mailed: Completed. This is more of a project management kind of report, showing how the small steps of the big project are being done on time and on budget (hopefully).
For your other phrases:
- "What we should do" -> Recommendations or Future Research
- "What we are going to do" -> Next Steps or Proposal
Next steps means you are committed to (and possibly have already started) doing the next things. Proposal or proposed next steps is what you want to do, but you are asking for permission and/or funding or something like that in order to be able to do it.
You could use Plans for "what we are going to do" but plans is more like what you are thinking you are going to do, it does not convey a strong commitment to actually doing it. You could use Follow Up if what you did was research and from what you have learned you see you now need to do more research and this is what you are going to do to answer the newly raised questions.
Side note: When I first read the title of the question, I thought of "What have we done?" as a somewhat rhetorical question about coming to terms with unintended consequences of an action. The people who created the atomic bomb, after seeing what it did to Japan, said to each other "What have we done?" I don't know a good word for that.