Source: I've been here 4 years, pretty much always in Shenzhen.
1) If you will be using wifi/landline internet (vs. always tethering to your phone), get a paid VPN that advertises working through the GFW and make sure it works before you leave. YMMV, but I've tried SSH tunnels, shadowsocks, lantern, streisand, and probably a few other open/DIY methods. In my experience none of them work for more than a day or so before they start getting throttled or completely blocked. If you are constantly moving from one connection to another it might be different. Your results could also be different in a different area of the country.
2) Get a working foreign SIM that lists China somewhere in the contract, or buy a Hong Kong "cross-border" one (not a generic HK one, which will only work in HK)... they won't be blocked. The Hong Kong one will likely only work in Shenzhen and Guangzhou, but it's probably cheaper than setting up something based on a US SIM.
3) Don't worry about your laptop/phone unless you are a journalist or work at a target for industrial espionage (for example, FAANG)
4) Don't go out of your way to start controversy, but certain people can have interesting conversations about sensitive topics. Give it a little time and ease into it if you feel like you need to talk politics. Or if you have trouble reading people's emotions just skip it. Pretty much anybody with a graduate degree in a technology field would probably be happy to help you debug VPN issues if you're already friends with them.
And some not so much security as general survival topics:
5) "Boo yow lah jhaow" = quit putting those fucking Sichuan peppers in every single thing I order (YMMV in different areas of the country, but here in Shenzhen even cabbage isn't complete unless it's fried with a generous handful of peppers and vegetable oil). (This will be completely ignored about 50% of the time, even if your pronunciation is correct.)
6) Baidu maps is pretty helpful if you want to show a cab driver your destination. They won't be able to interpret google maps. Older cab drivers won't be able to see the small characters on your screen, but if you take a screenshot then zoom in sometimes it works. Or just choose young cab drivers.
7) Wechat. Everybody has it. Even if you never try to pay for anything with it, you can quickly "friend" someone and even if neither of you are dual language you can long-press on messages and use the in-app translation to get a lot done. Just be careful to avoid idioms and slang, use simple sentences, write out pronouns instead of dropping them, etc.