The best thing is to slightly change how you are looking at it. Think connection rather than stimulation. Building confidence and self esteem is the best way to help a baby develop and learn. A child that is confident of themselves and you will be more willing to fail, which is really important to learning.
So build your connection to your baby as much as you can. Soon to skin time s.g. having a bath together is great bonding and a sensory experience. Carry/wear your baby as much as possible and talk and sing to them.
Let them try out their body in a range of different positions e.g. on their tummy, in their backs, propped on their side, held in different positions by you, over your lap etc etc. Change their position every 5 minutes or as soon as they seem uncomfortable - learning is not achieved when unhappy!
Smiling and talking and lots of eye contact. Flash cards are ok, but really only because it's time you are smiling and facing your baby.
Expose them to the real world. Nature, shops, toddler groups. Variety is big for you, but make sure your baby has adequate quiet and relaxation time. It's all new to them and they have a lot to process.
Relax if you can. It's hard when you halve a small baby - you want a tick list so you know you are getting it right and doing your best for your baby. But too much of that attitude will cause problems - stress on you (possibly burnout just when your toddler ACTUALLY would benefit from stimulation!), stress on your baby ( which means they learn less well and could become anxious), a baby that has to g "entertained" rather than one that can learn from their own experiences (which is really tough when they are a toddler and don't nap so much and you NEED to hang the laundry etc).
Connect. Don't entertain. It's an important difference in the long run so get into good habits now.