Everyone's covered it well. I love Macs but Stens definitely have more character.
-Get a loop stock if it doesn't have one. The original T-stock is bad!
-Get a pistol grip just for the variety. I love shooting mine either from the hip with the pistol grip and original barrel or with the loop stock and suppressor and actually using the sights.
-A can is a must as already mentioned
-Find a mag loader you like. I like a simple tapco plastic push down type better than the original ones
-Find a copy of Peter Laidler's "The Sten Machine Carbine", a very entertaining book on the history
Spare parts are fairly available and some even currently in production like barrels, extractors, springs, etc. Don't pass up these good times to get a few spares for down the road. I noticed last night I have a half dozen bolts sitting on a shelf but only one has an extractor installed, so I'm ordering some parts myself. Edit: just found 3 extractors/pins/springs, so ordering a bit less, but $100 worth of spare parts now can be priceless later.
Mine likes lots of lube (even with an np3 plated bolt) but eats absolutely every type of 9mm. If I find a to-good-to-be-true deal on ammo only to find that flat points won't feed, etc in other guns it gets set aside for the Sten, which just doesn't care.
Edit: This is the pistol grip I like. It is a bit fat as it comes but I've just squeezed them down a tiny bit in a vise to fit more normal hands and it has a loop useful for a single point sling.
https://www.sarcoinc.com/sten-mk2-mk3-commando-grip/