[SalesForce] What fields determine what is displayed in Open Activities vs. Activity History

The related lists are not in your configuration for that object. Edit the layout, re-order the related lists, click "Save," and when the "Override Users' Related List Customizations" appears, click "Yes." All users will now see the related lists.

It's a bit of a hack, but you could create a Visualforce page and controller action that will redirect the user to the most recently created Task for a record.

Set the &retURL going into the task creation to send the user to this new VF page and include the parent opporutnity record ID in the query string.

/00T/e?what_id={!Opportunity.Id}&retURL=%2Fapex%2FredirectLastTask?rId= {!Opportunity.Id}

public with sharing class redirectTaskController {
    string recordId = '';

    public redirectTaskController() {
        recordId = ApexPages.currentPage().getParameters().get('rId');
    }

    public PageReference redirectToTask() {
        PageReference activityPage = new PageReference('/' + recordId);

        Task[] recentTasks = [SELECT Id, CreatedDate 
                              FROM Task WHERE WhatId = :recordId 
                              ORDER BY CreatedDate DESC LIMIT 1];

        // Check that the new Task if fairly recent.
        if (recentTasks.Size() == 1 && 
                recentTasks[0].CreatedDate.addHours(1) > Datetime.now()) {
            activityPage = new PageReference('/' + recentTasks[0].Id);
        }

        activityPage.setRedirect(true);
        return activityPage;
    }
}

Visualforce Page

<apex:page controller="redirectTaskController" action="{!redirectToTask}" >
    <p>Redirecting to task</p>
</apex:page>

Another idea would be to create a Visualforce page that uses the Task standardController and provides a custom save action.