I, personally, would go for two simple and small items; one of which reminds you of them (flowers, brownies/dessert, toy related to their interest), and one music related (music box, vinyl record with at least one song you can both dance to, or one of those cards that play a tune with enough space for you to attach a favorite photo of the both of you inside). The amount is typically more than sufficient without overdoing it, and demonstrates how much you think of them while letting you save the expensive stuff for anniversaries or birthdays.

If you're wondering about for guys specifically, try food or flowers. Us guys don't usually get that kind of show of affection in the work place like girls often do and would really appreciate it. Especially if you show up in person to deliver it. Otherwise, that asshole coworker of his, Stuart, will crack a joke about him sending it to himself and then he'll have to spend the rest of the day being mocked and laughed at while trying to assure that his SO is indeed real, which will make them doubt even more, so then in a fluster, he'll attempt to prove it with pictures on his phone but NOOO, everyone is too busy snickering or busy working to even glance at the evidence held up next to their face, so he ends up dreadfully spending the remaining day pawing at his workload like a kicked puppy in anxiety that that dumb Stuart is going to say something to add on to the mockery.. not that any of that is from personal experience. But yes, simple gestures like that will really make a guy's day.