Because it's open wifi, it means it is possible for others to eavesdrop on your communications. It's no different than your wireless network at home should you leave wireless security disabled or use the weak WEP security protocol.
Minimally, you should define the open wifi location as a “public” network so Windows will use the strongest personal firewall settings. While that protects the machine itself (e.g., others can’t see or access your shared folders or files), it doesn’t protect the communications from and to your computer.
As long as your communications use secured protocols, it's reasonably safe. For example, any time you're using https://, all your data is encrypted between you and that website. In contrast, http:// is not secured. Other protocols for ftp, telnet, etc., likewise have their secured cousins (e.g., sftp, ssh).
Admittedly it’s a bit awkward to manage individual websites/protocols in this fashion, or monitor changes from http to https and back within the same website. You spend more time worrying about what is and isn’t safe than just getting your work done.
The best thing you can do is connect to a VPN server, preferably your own back home, so that ALL your communications are encrypted in the area of greatest vulnerability; the open wifi location itself. Then you don’t have to worry about what is and isn’t safe to do. EVERYTHING is encrypted, from soup to nuts.