We're an informal bunch. Wear whatever makes you comfortable, but you can't go wrong with business casual.
There's no dress code at INBOUND. We've seen more attendees in sparkly cowboy hats than in suits, but you're welcome to dress up (or down) as your heart desires. Wear whatever you're comfortable and happy in.
Speaking of comfort, bear in mind that you'll be doing a lot of walking during INBOUND, so it probably isn't the occasion to break out your most show-stopping heels. You should plan to wear comfy shoes or sneakers for most of the week.
It would also be wise to dress in layers so that you'll be comfortable both in heavily air-conditioned session rooms and when you're outside enjoying the sun during lunch.