If you do not want to fight with missiles...
Electricity and gas a weapons. Used by both sides too (Ukraine threatening transit and Russia threatening supplies on a daily basis).
Where Ukraine did not have the capacity to use computer attacks it has used dynamite: http://www.bbc.co.uk/news/world-europe-35204304.
They also did it first and pretended to "investigate" so this looks like a tit-for-tat retaliation.