I'm an executive assistant with reports to all 5 senior staff members. I never get anything on administrative professionals day and I'm fine with that. Last year my co-worker tried to get me to go in with her for a bosses day gift, but I think that's the most ridiculous thing ever. I'm cool with not doing anything for either "holiday".
Before this job (my 3rd year here), I was a legislative assistant for 1 year and a non-profit admin assistant for just under a year. The state rep I worked for was the only one who gave me something on admin professionals day, and he got me a nice potted flower for my desk which I did really appreciate. It was a really stressful legislative session and we'd had a lot of threats to the office, so it was especially nice at that time to get some recognition. I was also only making 25k a year with that job and I make 43k now, so maybe that was part of it.
At the office now, I usually get special christmas gifts (above the organization-wide gift card) from three of the senior staffers and another high-level manager that I work with 1/3 of the time. However, I also have to contribute $ towards gifts for the CEO and CFO at the holidays.
If you've got a good relationship with the assistant and/or if they've really been an asset on some projects, I think a note in email or card form thanking them for their work and saying how much you appreciate their determination/organization/resourcefulness would be a nice gesture. If you want to add a small gift card or their favorite candy bar, that could be nice without establishing a ridiculous precedence.