For Register of Probate: write in Sean O’Donovan, 30 Warwick St, Somerville (though just "Somerville" enough of an address). The effect of this vote, in the likely case that Buonomo wins the primary and then withdraws, would be to influence who the party committee picks as a replacement.
For Senate: Ed O’Reilly. Not that he has any chance of winning, but this is a good way to let Kerry know where his constituency stands on things like single-payer healthcare.
I don't usually make endorsements for districts I don't live in, but if you're in 34th Middlesex and care about progressive politics at all, I heartily remind you to write in "Carl M. Sciortino, Jr. 17 Orchard St, Medford" for state representative.