From a developers perspective, if I'm not actively looking for a job I almost never want to talk to companies that are trying to recruit. The one time where I actually went out of my way to talk to a company was when they held their own event. They rented a space with an open bar and brought their engineers with them. You could just go hang out and talk with the engineers and see what kind of stuff they worked on what not and it was a great way for us to vet the company and decide whether or not we could see ourselves on the team.

Swag is always good. Stickers are a given.

One conference I attended had a company handing out cards with a puzzle on it (encoded message). Decode the message and email it to them and they tell you to drop by their booth for a free t-shirt. Clever way of collecting email addresses for recruiting and they mention that they're hiring when you get the shirt. They never did spam me via email, so I didn't think it was too intrusive.

When you say swag, can you give 1-2 examples in mind?

The puzzle card sounds a great idea. Do you remember roughly what the puzzle was?