There is no such thing as 'secure' on the internet.
I'd say a laptop booted from a read-only live CD from a random open WiFi access point would be pretty secure. They could find the access point and get security footage, but you're asking for trouble if you're opening your laptop and working on whatever at the place. I'd write out everything ahead of time and set the laptop not to turn off when the lid's closed. Set a script to email/post shit as soon as you connect to an open WiFi. Then leave that shit in your backpack and just stroll on by.
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