Via a Facebook event created by Christian L. Frock:

Protest and disrupt business-as-usual from a distance. Give form to the vast digital community that demands Ai’s release: In honor of Ai’s “Sunflower Seeds,” send seeds to the Embassy as a gesture of protest, from home or online.

100 seeds + standard envelope + single page letter = $0.64 US Domestic Postage or 2 First Class stamps

Find seeds: groceries, liquor stores, hardware, garden/pet supply

Or buy online and ship direct to Embassy:
Chinese Embassy
Attn: Mr Zheng Yesui, Ambassador
3505 International Place, N.W.
Washington, D.C. 20008

Examples here.

Sample letter of protest:
I am writing to express my concern about the ongoing detention of artist Ai Weiwei. It appears that Ai’s detention violates international law. There is no justifiable reason for his sudden disappearance and the lack of information provided by the Chinese government concerning his whereabouts and condition. Ai is widely regarded as one of the most important artists and cultural figures of our time. His detention sends a negative message to the world about the China’s commitment to human rights, freedom of speech, and due process of law. I respectfully request that the Chinese government immediately release Ai Weiwei.

>>>> OUTSIDE THE U.S.
Recreate this demonstration: organize appropriate information and go viral. Provide Embassy mailing address, sample text, sources and correct postage details.

Learn more about Ai’s detention at freeaiweiwei.org