Quality cogs make a world of difference...EAI , All City, or something of that ilk.
And get a couple of different tooth counts.
Quality chain rings are a blessing.

A new saddle.

If you ever plan to actually rando, get a good quality saddlebag for your gear.
I use a Dill Pickle bag which has served me for 3 seasons.

And as others have mentioned, a good, strong, light wheelset is a great purchase...just save the old wheelset! You never know when it will come in handy.

Cyril