Here are some ideas (and US prices):

1) Nespresso coffee machine. Can get a good one for < $200

2) Autonomous standing desk. Base model $399

3) Trackpad. Nice addition to the mouse. E.g. $129 from Apple.

4) Second monitor. I personally find this convenient. A decent one will cost you under $300.

5) Tablet. Useful for when you are too tired to sit or stand. Good for reading docs or whitepapers, for example. $329 for a base model iPad.

6) Faster internet. A few bucks added to your monthly bill.