On the job learning is awesome!
Before you meet with your boss, it would be a good idea to have a bit of a brainstorm about what it is you’d like to learn first so that you can discuss something more concrete with them. I’m guessing there’s a lot of interesting things you’re keen to sink your teeth into but you’ll have to choose a starting point at least
You could approach this by listing tools/techniques/methodologies you haven’t tried yet and seeing if anything stands out to you right now.
Have a look around your team and see if there are any projects that other people are working on that interest you and that you think you might be able to add value to by joining and gain value from through learning.
You could look at this from a skills gap perspective - either to broaden your skill set by learning new things or deepen it by gaining more experience in things you’ve already touched upon. At this stage in your career, I don’t think there’s a right or wrong place to start - you could even just pick something you like the look of and try it out!
Talking to your boss about it is great but like you said you’ll also have to take initiative and drive your on the job learning yourself. I’ve been where you are and learned that asking for on the job learning and ‘stretch assignments’ wasn’t always enough. I needed to actually pursue and create those opportunities for myself by offering to help out with other projects, taking on observer roles in other people’s research projects to experience that different perspective and I also started blogging about my experiences and thoughts on UX which opened a lot of doors that helped me grow and still does. I’m self-employed now and it can all be traced back to one article that I wrote 4 years ago.
When talking to your boss, you’ll also need to consider and communicate what they’ll get out of helping you grow in the way that you’d like to- what’s in it for them? (beyond having a more skilled employee). Look at the goals of the team and the organisation and work out how what you’re looking for aligns to that. What are their plans for the future and what part can you play to help them get there?