The key to promotional items is to be front of mind at the time a potential client needs your service... In your industry, if people already use you, I would be looking at things that are useful to them and add value - branded boxes / packaging or tape etc as part of the service - but give these away at point of contact, rather than at expo.
If you must give stuff away, because the stands that are around you are - drinks and mint sweets work well (lots of talking = stinky breath

) and people like a bottle of water - it can be branded, maybe have a glass fronted fridge full of them and your logo in vinyl on the sides. What are you known for - if it is speed, maybe a sports bottle would be appropriate?
You could try little brown cardboard boxes, stacked to look like parcels, with Fragile / logo and contact stamped on the side, with a treat, perhaps with a branded mug inside - creating relevancy and stickiness)
If this HAS to be a sticky item, and therefore be on someones desk for at least 3 months, staplers and calculators are good. They have retention and perceived value and your potential clients may well use them before they fill in Pro-formas / docs that are a requirement of sending . Other standard items like USB's do get collected, but as digital storage is cheap these days, I am not sure they are premium or used so much anymore.
If this is a brand building exercise, then I think the minivans you have are cool and quirky, but will probably be given to the kids when your target gets home...
Hope that helps!