How to get more free credit ?

As a new AWS customer, you can earn additional credits beyond the initial $100 you receive at account creation. You can earn up to an additional $100 in credits by completing specific activities designed to help you learn AWS services.

These activities are available in the "Explore AWS" widget on your AWS Console Home dashboard. The activities include:

1. Launching an Amazon EC2 instance - Create a virtual machine in the AWS cloud and learn how to manage instances
2. Using Amazon Bedrock playground - Experiment with foundation models by choosing a model and submitting prompts to generate responses
3. Setting up a cost budget with AWS Budgets - Create a budget to monitor your AWS costs and usage, and receive alerts when you exceed thresholds
4. Creating a web app with AWS Lambda - Build a simple serverless web application using Lambda functions
5. Creating an Amazon RDS database - Launch a relational database and explore basic configuration options

Keep in mind that while these activities help you earn credits, they do incur AWS service charges that will be deducted from your Free Tier credits. To maximize your learning while managing costs, make sure to track your usage and clean up resources after completing the activities. You also have access to over 30 AWS services that offer monthly free usage limits as part of the Free Tier program.
Sources
Earning additional credits - AWS Billing
Explore AWS services with AWS Free Tier - AWS Billing

I just created a RDS to start deploying my appplication and I see costs incresing, isn't it included in the Explore AWS, so cost-free?