I know you said in another comment
I didn't try looking at pond water or similar with it to find living things, only plant life so far.
But you really really should. Maybe I'm biased but to me, that is one of the greatest joys of using a microscope is watching the teeming activity of creatures that have always existed all around you but that you've never seen before. It's like walking into an alien forest and seeing the rich ecosystem there of fantastical beings and their interactions with each other, only these things have been all around you all your life, they're not alien, they're more common than squirrels, they just operate at a different scale than you do so you have no common knowledge of them. It's like being a newbie bird watcher, but again, in a new dimension. You start to notice patterns, creatures of similar type but that are also clearly different, analogous to different kinds of birds, from various songbirds to corvids (shoutout to @corvidenjoyer@hexbear.net) to birds of prey. You'll get to know types of diatoms that are (no offense) far more varied and exciting than the garlic skin in your OP image, and rotifers making convection in the water to suck up food, ciliates waving their (silly) cilia about, tardigrades (water-hexbears) all over the place. And then you can increase the magnification (depending on your microscope to some extent) and take a look at a whole new level - bacteria.
Experiment with different samples. See what you can find in different kinds of environs. It's awesome to see anything up close, but ime there's much more of a sense of discovery when you spot lifeforms, especially ones that do stuff you get to watch.
Nope, no need to stain. I actually haven't really looked at seawater much to be honest, but you're definitely going to get a different set of lifeforms in seawater than you will from fresh pond water. Making comparisons between them might be a good thing to record in your notebook, which is a great idea btw. In the sea water you might see some tiny crustaceans like copepods but not likely to see rotifers for example. Could see tardigrades in either, though!
In either case, pond or sea, you shouldn't need to worry about staining anything. Most of these things have their own pigmentation, especially diatoms. The critters will tend to look semi-transparent or transparent but with very distinct edges, something like this so no, no need to stain. In fact you shouldn't stain anyway because it will likely kill the creatures in your sample so you won't see their activity.
As for the terrariums, awesome. There is a lot to be considered there, but what an amazing way to truly delve into those questions. If or when you do set something like that up, I hope you post about it here. I'd love to read about your findings.