Here's a much closer version to what you're trying to achieve, but I feel I'm still missing some pieces.

trigger UpdateCEPfromOpp on Opportunity (after insert, after update) {
	if (CEPTasksFromOppHelper.hasAlreadyCreatedFollowUpTasks()) {
		return;
	}
	Set<ID> idAccount = New Set<ID>(), idOwner = New Set<ID>();
	Map<Id, map< id, set< Id > > > oppKeys = new map< id, map< id, set< id > > >( );
	Map<Id, Customer_Engagement_Plan__c> plans = new Map< Id, Customer_Engagement_Plan__c>( );
	
	for (Opportunity Opp : Trigger.new) {
		idAccount.add(Opp.Accountid);
		idOwner.add(Opp.OwnerID);
		if( !oppKeys.containsKey( Opp.AccountId ) ) {
			oppKeys.put( Opp.AccountId, new Map< Id, Set< Id > >( ) );
		}
		if( !oppKeys.get( Opp.AccountId ).containsKey( Opp.OwnerId ) ) {
			oppKeys.get( Opp.AccountId ).put( Opp.OwnerId, new Set< Id >( ) );
		}
		oppKeys.get( Opp.AccountId ).get( Opp.OwnerId ).add( Opp.Id );
	}
	plans.putAll([Select Id, Account__c, OwnerId
											from Customer_Engagement_Plan__c
											where Account__c IN :idAccount and OwnerID in :idOwner AND (End_Date__c == null OR End_Date__c > TODAY)]);

	list<Task> tasksToUpdate = New list<Task>{};
	for(Task Tasks : [Select id, accountid, CEP_Activity__c, whatID, Subject, Status, Ownerid, Description, ActivityDate
					  from Task 
					  where (WhatID IN :plans.keyset()) and Status <> 'Completed' AND ActivityDate <= NEXT_N_DAYS:14]) {
		Tasks.Status = 'Completed';
		Tasks.Description = '';
		Customer_Engagement_Plan__c cpe = plans.get( Tasks.WhatId );
		for( Id oppId: oppKeys.get( cpe.Account__c ).get( cpe.OwnerId ) ) {
			Opportunity o = Trigger.newMap.get( oppid );
			Tasks.Description += 'Opportunity ID ' + o.SFDC_Opportunity_Unique_ID__c + ' was updated. Close date is ' + o.closedate.format() + ' and Estimated revenue is '+ o.Estimated_Revenue_Amount__c + '\n';
		}
		tasksToUpdate.add(tasks);
	}
	update tasksToUpdate;
	CEPTasksFromOppHelper.setAlreadyCreatedFollowUpTasks();
}

Here's some problems you had:

1) You were querying the entire list of opportunities from the trigger per each opportunity. This is doubly redundant because (a) Trigger.new already has all the information you need, and (b) you wouldn't need to requery the data each time.

2) Many useless variables. Sure, it's great to have a variable called dtToday, which is shorter than System.Today(), except that you didn't even use it consistently, so you'd make people wonder if there was some magic going on. Same thing for dtDisplay; the result is you're trying to make a shorter string of code at the expense of more code.

3) Filters. You can tell that the code I wrote is more efficient because it asks the database only for tasks that match records it actually intends to process. The code have fewer rows returned, and the code gets to cut out irrelevant if-then statements as a bonus.

4) Your code wouldn't work right when using the data loader, for example. This is because the opportunity/task/cpe data would get all jumbled together; you don't actually bother to keep track of the associations, so it would fail to update all the tasks with all the correct opportunity information. In my example code, we we "maps" to keep our data organized. Note that my code precludes the possibility that a description field might notate that multiple opportunities were the catalyst for the task update. This isn't strictly necessary, but it might be useful to know.

5) I have a feeling that SFDC_Opportunity_Unique_ID__c is actually a formula for "Id". If so, you probably should just use Id instead. If it's an auto-number, that might not be the best idea, for pratical reasons (someone could reset the auto-number index and confuse people).

6) Ditto for Estimated_Revenue_Amount__c. Salesforce already has a field that does something like this, so this might be redundant. But, I know many organizations don't like the default definition of "estimated revenue" (probably because they don't understand its purpose), so if that field makes sense, then feel free to use it. Just an observation.

7) I'm not sure you really, really meant to query based on OwnerId/AccountId, but I built this code with that assumption. This sort of query sets of warning bells in my head that something might be amiss. Of course, I could be wrong, and if it helps you out, great.

8) Things might still be broke in my example above, but I hope it helps you get where you're trying to go.