Serious Games are games with a higher purpose than pure entertainment. But how should you balance the serious elements in your game?
At first you have to distinguish between the main target or motivation of the person who finances the serious game (further called "manager") and the main target or motivation of the user.
The main target of the manager can be to raise awareness on a certain topic, to improve the learning outcome of students or employees, to increase sales or sneakily manipulate the target group. The motivation of the manager is normally this what is hidden behind the word "serious".
The main target of the user can be the same target as the manager's target, for instance getting informed about a certain topic. At the same time, the user can enjoy the format of this learning scenario, so a combination of learning and fun. But the main purpose is still to get informed.
On the other hand the user's main target might be the search for fun and entertainment. This is especially true when the user is not aware of the fact that she/he plays a Serious Game.
Let your users explore your game and receive some elements of "serious" from time to time. The best way is to include it into your game is to really make it part of the game. It is a bad game design if there are different sequences of fun followed by sequences of seriousness. Your user will directly recognize your trick and will be disengaged. Integrate your element of seriousness or your learnings directly into the gameplay.
To deliver a message to a user audience who has the main target to seek for fun and entertainment, you have to be careful not to put too much of "serious" in your game. In my projects in development cooperation i found out that a good rule of thumb is 70% fun and 30% seriousness.
This number changes if the main motivation of the user corresponds to the main target of the manager. The more their main motivation overlapps, the higher you can go for the percentage of serious. For instance: If you want to teach your employees how process XY should be executed in your company, you don't need to make this learning very subtle or hidden. You can go for direct messages and change the balance to 50% fun and 50% seriousness.
Don't forget that you decided to develop a serious game, so including fun is essential and will also improve the outcomes. If you finance a game that is not enjoyable, people won't play it. If you have to force them and they don't enjoy it, then it's not better than a very expensive pdf document.