Knock Box will make life a bit easier.
Having the right milk jug will help with latte art.
Stock tamper is usually light/flimsy, so upgrading that would make the coffee making experience a bit more enjoyable. Get one that fits your basket.
Filter baskets - even though Ive got an after market filter basket, im not convinced that theres a huge improvement. Perhaps its my technique, but I went for it purely because it was a toy and I wanted to experiment with different things.
Roaster - something to look into; not essential but it is an associated toy. Again, great if you like having first hand experience with all things coffee.
- A cheap paint brush for cleaning mess from the grinder
- Shower screen brush to scrub the shower screen and the seal
- detergent for backflushing