Hi Ramon, I think this came up on another thread. Chargers seemed popular and sports bags. I think the key is knowing your audience and giving them something with a perceived value which they can utilise in the very near future so they remember you/make the association. Alternatively you can opt for something that will give you good stand out and if it lends itself to sharing on social media channels so much the better.
I've seen tote bags work really well as people often end up with lots of literature and other giveaways from exhibitors to carry and haven't brought anything to put it in. Plus now we're all trying not to spend 5p on a plastic bag reusable one has good appeal and if you create a good design that people want to be seen with then suddenly your bags are all around the event giving you excellent exposure. At another event I was at an agency were giving away inflatable dinosaurs which proved popular... I carried mine all the way back to Kent and into the office the next day which then became a talking point and here I am mentioning it about four years later.