Hi everyone.
Can I get some help on other things I should do while I am taking many other parts off the engine (bul not pulling the engine)?
The car is a 1999 Impreza L with just under ~100k miles.

Here's what I am doing so far:
I am pulling the intake manifold to drill out and rethread a PVC valve that sheared on me. (Sometimes I feel very unlucky with cars :( ).
I figure I will also replace the timing belt. I remember reading at sometime something about Subaru's being prone to leaking front seals and also timing belt tensioner replacement. Should I just go ahead and do those too?
My spark plug holes have oil in them, so I will remove the valve covers and replace all of those gaskets. I will also check the valve clearances while I'm in there.

What else should I do while in there? Am I missing some obvious things that just make sense to do at this point? Any advice would be much appreciated. Planning on putting my parts request in today and order some extra tools I will need so, hopefully, I can finish up over the weekend.

If you want it to go another 100k+ then check the valve train for more than just clearance and check everything, then replace as needed (guides, seals, etc.). I'd also check the rings to make sure everything in the bottom end is happy too.

^^^
Will do. Can I check the rings and bottom end without pulling the block though? Sorry if that's a dumb question. I'm learning all of this.

You can visually see the rings/pistons when you pull the heads.

You can do a compression test, tool is about $40, and that should give you a decent idea on the ring condition. As for the rest of it (guides, valve seal replacement) you usually have to at least get the heads off of it, and at that point you have to start weighing out more options such as just swapping to a low mileage or new motor based on cost/benefit. If it isn't burning any oil chances are the guides/seals are good. A leakdown test is also a good idea though it requires a little more know-how. I agree you should do the timing belt/tensioner/cam seals/spark plugs gaskets though while you're there and maybe a water pump.

Thanks all. It's coming along- though very, very slowly.
I got the intake manifold off last night and started in on that.

Interestingly, I discovered a broken spring laying under the manifold.
Apparently, it is from the clutch assembly and based on the rust at the break it's been there awhile. So now I have something new to look at a bit closer.

Was the clutch ever replaced?

^^^
Yes it was almost 50K miles ago though.
From what I can tell it originally attached at the engine block and the clutch lever.