Small handful of nuts
Walnuts, almonds, or two to three Brazil nuts and no more. A small handful is the portion; a bowl on the desk becomes three handfuls before lunch.
Apple and peanut butter
Cut the apple into wedges and dip. It takes two minutes and it stops the 4pm biscuit run without any willpower involved.
Green tea, swapped in
In place of one fizzy drink or the fourth coffee of the day. Green tea's role here is substitution, not magic: you are moving one drink, not adding a supplement.
Containers, not packets
Keep the portions small and the bag out of the desk drawer. A small container works better than a large one, and that is a logistics point rather than a nutrition point.