The stimulation has nothing to do with pushing academics. Do you talk to him? Read to him? Take him places? Do things with him? Let him do some unstructured and self-directed play both indoors and out? Have him interact with a variety of people and ages? Let him take risks? Has he fallen down and collected a few bruises? Has he ever gotten spectacularly dirty? Has he played with water and dirt and sand and grass and sticks?
From your post, the answer is obviously "yes." You're doing fine.
Toddlers are stimulated by interacting with the world and people around them. The understimulated toddlers are the ones who are kept in "safe" environments, never interact with anyone other than parents, never go outside, and watch lots of "educational" videos. They're the kids who have never gotten dirty, never gotten hurt, and never learned how to move their bodies. They're the kids who have zero social skills because they've never actually had a non-family social interaction before. Their entire world is their bedroom, living room, kitchen, car, and parents. Some of them can count and recognize letters, but that doesn't mean they can function in the real world. In general, detailed plans to "stimulate" a child's mind tend to do more harm than good. Children stimulate their own minds in a rich environment far better than adults can do it in a controlled environment.
The window of "stimulating the mind" never closes. You don't have a deadline. Even adults are constantly growing new brain connections. Encourage his curiosity and share the world with him. Let him take risks. Let him try and fail and learn how to get up again. Celebrate his efforts, not his achievements. Remember that the simplest things (like playing at the park) can actually be some of the most powerful things for child brain development.
How do you know what to improve on? Well, if you notice any specific areas of weakness, make sure he gets some extra time to practice that. For example, if he can't pedal a trike (which is pretty normal at his age), make sure he gets lots of opportunity to play with a trike and figure it out.
But in general, relax. Play with your child. He's stimulating his own brain quite competently. Most people telling you otherwise are trying to sell you something....and it's usually not something good for you or your child!