This si my current Controller:

public class CalendarEventController{

public List <Account> getMyAccounts()
{
return [SELECT Id, Name, Region__c FROM Account ORDER BY LastModifiedDate DESC LIMIT 10];
}

public List<ActivityHistory> Actividades {get; set;}
public List<List<ActivityHistory>> lista {get;set;}
public List<List<ActivityHistory>> getOpen()
{

lista = new List<List<ActivityHistory>>();
SObject[] accounts = [SELECT Name,
(Select Subject, WhoId,WhatId, ActivityDate, Status, Priority, OwnerId FROM ActivityHistories) From Account];
/*for(Integer i = 0; i< accounts.size(); i++ )
{
if(accounts[i] != null)
{
Actividades.add(((List<ActivityHistory>)accounts.get(0).getSObjects('ActivityHistories')));
}
}*/
Actividades = (List<ActivityHistory>)accounts.get(1).getSObjects('ActivityHistories');


for(SObject o : accounts){
if(o.getSObjects('ActivityHistories') != null){
List<ActivityHistory> element = (List<ActivityHistory>) o.getSObjects('ActivityHistories');
if(element != null){
lista.add(element);
}
}

}

return lista;
}

}

As you may see im trying to get all the events from every account we have.

This is the apex code that im trying to use in order to display it on my visualforce page:

<apex:pageBlock>
<apex:pageBlockTable value="{!Open}" var="lista">
<apex:column value="{!lista.Subject}" headerValue="Subject"/>
</apex:pageBlockTable>
</apex:pageBlock>

I keep getting the same error: Error: Unknown property 'VisualforceArrayList.Subject'