Task relationship to Opportunity in sub query

I’m attempting to create a Batch class that will Stamp Opps with their Next Activity Date.

I haven’t been able to figure out the relationship between Opps and Tasks? I’ve tried: Task_r, Tasks_r, Activities__r, Activity __r with no luck

Error: Variable does not exist: Activities__r

Thoughts?
Code:

public class BatchNextOpportunityActivity implements Database.Batchable<sObject>, Database.Stateful, Schedulable{

//variable to hold any errors
private Set<String> apexErrors;

//List to hold Opportunities to be updated
List <Opportunity> oppsToUpdate = new List <Opportunity>();

//instantiate the class
public BatchNextOpportunityActivity(){
    
    Set<String> apexErrors = new Set<String>();
    this.apexErrors = apexErrors;

    List <Opportunity> oppsToUpdate = new List <Opportunity>();
    this.oppsToUpdate = oppsToUpdate;
}

public void execute(SchedulableContext SC) {
    Database.executeBatch( new BatchNextOpportunityActivity() );
}

public Database.QueryLocator start(Database.BatchableContext BC){
    
    String query =  'Select ID, Next_Activity__c,(ActivityDate FROM Activities__r  WHERE ActivityDate > Date,Today() ORDER BY ActivityDate desc) from Opportunity WHERE CreatedDate = LAST 120 DAYS';
    
    return Database.getQueryLocator(query);
}

    public void execute(Database.BatchableContext BC, List<Opportunity> scope){
    
    system.debug('scope.size = '+scope.size()); 
    
    for (Opportunity opp : scope){
        //If the ActivityDate of the 1st Task is greater than the Opps Next_Activity__c then let's update it with the Task's ActivityDate
        if (opp.Activities__r.size()>0 && opp.Activities__r[0].ActivityDate > opp.Next_Activity__c) {
            opp.Next_Activity__c = opp.Activities__r[0].ActivityDate;
            oppsToUpdate.add(opp);
        }
        
        //If there's no Tasks found and the Next_Activity__c field is populated lets clear it 
        Else if (opp.Next_Activity__c != null) {
            opp.Next_Activity__c = null;
            oppsToUpdate.add(opp);
        }
    }
}