There will be a camp for protesters, more infos : here.
You should come to Hamburg at least on 5th July. German Police is in high alert, they will check the borders to prevend protesters from coming.

This was the video of the anarchist call against G20 in Hamburg.

And this are some actions, collected from https://tschuess.noblogs.org/post/category/english-direct-action/

Stones against SPD and prison profiteur
Arson attack against a ThyssenKrupp-vehicle
Attack against ThalesArson attack against Deutsche Telekom
Excavator burns – against G20-Summit
Attack against construction-company HOCHTIEF and a call against the G20-summit
Attack against the ‘Messe Hamburg’, venue for the OSCE and G20 summits

Athens, Attack against the office of security company SCS
Berlin, Arson attack against a police station
Leipzig, 3 public-order-police vehicles torched
Sweden, Bank attacked
Leipzig, Arson attack against Job Center
Arson attacks against Sodexo and Vodafone
Berlin, French diplomatic vehicle torched
Bremen, Bundeswehr (Armed Forces) vehicle torched
Berlin, Arson attack on a police station against the Police Congress & the G20 Summit
Berlin, Securitas (private security company) vehicles torched
Hamburg, Two police cars torched
Hamburg, more Police cars in station torched
Arson attack against a ‘Securitas’ private security company vehicle
Madrid, Spain : Incendiary attack against the Cavalry Unit of the National Police
Bremen, Incendiary attack against Jobcenter
Arson Attack Against the Vehicle Fleet of ‘Deutsche See’
Weilheim, Arson Attack Against a Police Station

and this will happen :

there is a calender with the main protest events

International anti-capitalist demonstration against G20 summit : G20 Welcome to hell

If you can not come to Hamburg, ATTACK EVERYWHERE !