Familiarising yourself with the EU-HEART materials will certainly give you a lot of ideas on how to work across subjects. At least, this is what happened to the teachers involved in the pilot. Here is one of the many examples:
In Greece, in subjects such as biology, where the origin and type of plants with which Queen Amalia decorated the National Garden were investigated. Also, in Social and Political Education, a visit was made to today's Hellenic Parliament, which is housed in the building built as the palace of King Otto and Queen Amalia.