Are you thinking about actually exhibiting at the expos, or simply walking the floor and trying to network with fellow attendees? I've attended tons of these and worked one or two in my day.
Exhibiting can be a great marketing opportunity, but (for the expos that get enough traffic to be worth your while), in my experience you're going to have to have at least one other person, maybe two or three, with you to help staff the booth. You'll need to eat, take "necessary breaks" and otherwise have some down time during the day, and you don't want to leave your booth empty. You'll need a decent professional-quality display (at least on a par with the other exhibitors -- you want to stand out, but not necessarily as the one with the really awful, amatuer display) -- and you'll want to be sure you've got plenty of takeaway literature (brochures, cards, flyers).
And keep in mind, depending on the expo, it can be expensive to rent the space in the first place, so you want to choose carefully. It can be a significant investment of time and money, so you want to make sure the attendees are really going to be strong potential customers for your product/service. Sure, many business executives have children, but setting up a booth promoting the sales of Usborne Books at a business expo targeting CEOs is probably not the best plan.
You'll also want to confirm with the expo organizers exactly what they provide for exhibitors. Often they'll charge extra for things like an electrical outlet, a phone line, tables, chairs, etc. Don't assume that they include anything other than a few square feet of floor space. You'll want to double-check and confirm everything, then bring as much stuff with you as you can (power strips and heavy-duty extension cords, a couple of folding chairs, tables and covers, etc.) just in case. You want to be prepared to deal with any unpleasant surprises that might be waiting for you when you arrive.
Take full advantage of every minute they allow for set up/breakdown, and practice setting up your display several times before you arrive. It will
take longer to set up and pack up than you anticipate. You don't want to still be breathlessly trying to set up your display when the first attendees are streaming through the door, and you don't want to get in trouble with the organizers/venue by taking too long to vacate the premises afterwards.
While cute tchatchkes and contests/giveaways/drawings for cool prizes can attract a lot of attention and traffic to your booth, but I'm not sure whether they actually do anything for you in terms of actually gaining you more business. I personally have walked away from many expos with bags laden with swag, without buying anything from the companies handing out the trinkets. On the other hand, if they've got a product/service I need and their offer is good, I'll buy from them even if they haven't given me anything more than a flyer and a business card.
If you're planning some guerilla marketing by walking the floor and working the room, just be aware that some expos frown on that (they want to protect the market for their paying exhibitors), while others are okay with it, so you just want to be careful when/where/how you go about it.