I don't recommend buying a ton of Math materials for home use but there are a few that I have found extremely helpful and valuable. One of them is the numeral and counter set. We have used these a ton and it's still the little one's go to Math work. We started with a mat and the box of counters and numerals. Find the number one and say "this is one" and than put one dot under the number. At first you may only do up to three depending on age. Continue until you reach nine.
An example of what the flashcards look like. You can use these with the red counters or without. I've seen some fun activities incorporating clothespins as well.
You can easily to this with 55 similar objects and some cards with numbers on them.
Montessori Print Shop has some Summer Numbers and Counters for Free
Montessori Album demonstrates how to introduce Numerals and Counters in a lot more detail.
This week I introduced this same lesson but with the graph paper cut out so there is one spot for each dot. This helps makes it self correcting and is a great way to start playing with numbers. Yesterday we worked on "how many more?" to get to our target number. "I Spy" is another fun game to play with either the counters or the numerals.
Did you notice we skipped a few lessons? In a perfect world you would introduce the number rods and the spindle rods first before the counters. You know your kids though and many of them will do just fine going straight to the counters after learning one to one correspondence.