I understand the bored sentiment, and sadly in some situations it is true. Steps can be taken differently though, doing “horizontal” enrichment (things not taught in school) ir advocating for the child. It’s just less of a problem in Montessori.
With that said, I am doing what you are doing. 
I have a 4 year old in Montessori and I supplement at home. One thing I’ve done I wish I hadn’t was having Montessori materials at home. They were no longer novel at school, and his attitude of already knowing how to do it was not all that appreciated. (I’m not advocating for not supplementing though!). I just wish I hadn’t used the tools someone else was using with him elsewhere. I did not give him an edge. Perhaps if he were more mature, or more compliant, instead of an active boy, he would have just been able to demonstrate his knowledge and move on. It’s mostly straightened out now, I just won’t be doing that again with my second.
Is your child in a toddler program or primary? Perhaps going into primary next year? That would definitely give so much more in terms of education.
What we are doing right now, and basic schedule:
Calendar/Geography lesson upon wake up (home-made, just look at the day, read what’s on the agenda, and talk about and locate one state each day)
Math worksheet at breakfast table (we are currently using 1st grade Jump Math workbook, but will soon change to Jones Geniuses)
Memory work at breakfast table as a family. (using CM method)
School (listen to Suzuki music in transit)
Home from school, we have snack and he does Handwriting without Tears program (I really cannot say enough good things about this program). He’s almost done with the pre-k version, and then we will move on to the kindergarten workbook
We then either play word or phonics games. Sometimes I print out words for him to decode and pictures and use the brillkids binder. I’ve also got a lot of other games. Sometimes we play go fish or memory with phonogram cards. Some days we don’t have time or one of us is grouchy and we skip it. I’m using the curriculum “Rocket Phonics” and adapted it a little for him. We are about to ramp up in the summer though, I’m excited! I would like to try All About Spelling when we are done with this program (which is a “light” phonics program), and go through all the levels of AAS to give him a thorough phonics education. (I never did any whole language learning with him, but am using little reader with my second, I will have to see when we get there for her which phonics programs we use).
Then, he can pick a quiet activity (legos, modeling clay, etc.) and I do a read aloud. Currently, I pick a book and then he picks a book.
After that, on days he doesn’t have any extracurriculars, I encourage him to play in the back yard for a bit. Or, he’ll play with whatever he wants inside, or he’ll follow me around…
We’ve just stopped (or are taking a long haitius) from Suzuki violin, but if your child is receptive I highly highly recommend it. I’m looking into private percussion lessons instead, as my child is much more interested in percussion instruments. We haven’t ironed the details out yet though. Some after school music instruction or class may be fun though.
He has Tae Kwon Do twice a week in the afternoon, and a science class at the library once a week (that’s stopping for the summer though).
At dinner, I sometimes have more Jump Math or Kumon worksheets, if he wants to do them. He also sometimes watches an educational video (leapfrog, preschool prep sight words, bill nye, readeez, etc.), but moreso on the weekends. We usually don’t have time on the weekdays. I some times flash him EK bits I have for my daughter (he actually really likes them, I could probably do more, but they are honestly of low priority).
After dinner and before desert, he reads a book to us, and we keep inching up the level. He’s currently going through the BOB books, though they are quite easy for him now, he gets a new one each day. When we’re done with them, I plan to purchase the Usborne graded readers and work up those each night. We add a paper link to his paper link chain in his bedroom for each new book he reads independently. While he eats dessert, I read some more to him (this time, it’s usually picture books, his choice). Then it’s bedtime.
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