I'm still in "baby steps" mode for saving seeds. I have saved okra seeds for several years now. It's normally slow to come up and likes warm soil temperatures to germinate. Tried something new this year. I transplanted a very long row of them that I had started in the greenhouse but had a dozen or so holes where I needed more. So I took some seeds, put them in water in a small dish, and put that on top of our water heater. In 36 hours, they were just about all showing a tiny little sprout happening. I took them directly to the garden and put them in their little holes. I think it took a few days, but nearly every one of those little babies came up! So I think next year, I'll do the soaking in water on the water heater trick again. Will probably save a week of waiting.
I've saved sunflower seeds. I've saved pumpkin seeds. I've saved pepper seeds. I've saved some bean seed of several kinds. I've saved a lot of tomato seeds. And maybe a few more that won't come to mind. A few that I really would like to save are brassicas (cabbage, cauliflower, broccoli, Brussels sprouts), pea seed, carrot seed, squash seed, corn seed (I wanna try some open pollinated, there are a couple of varieties), spinach, beets,... I know, lots of possibilities. And I know one has to be mindful of cross pollination on some stuff way more than other stuff. I'm likely to run out of time way before I run out of stuff to try.