The most visual, most fun way to learn decimals is with decimal squares.
We use decimal squares we bought from https://decimalsquares.com several years ago. 
They show decimals in tenths, hundredths and thousandths, and the equivalencies are obvious. (see below)
The website has fun interactive games, too, but you need to download the Shockwave app to play them.
ACTIVITY ONE – The “I HOPE I GET…” Game
For 2, 3 or 4 children.
Materials Needed: Deck of decimal cards.
To play: Shuffle the deck and lay face down on the table. Player #1 turns over the top card and lays it face up on the table. She looks at the card and says which card she hopes to draw in order to make 1 (For example: if she sees 3 tenths, she says “I hope I get 7 tenths!”). She draws a card. If she can make 1, she tells the other player which cards add up to 1 and takes those cards. If not, she lays her card face up to the table top. Suppose she draws 5 tenths — then she has to lay it on the table.
Player #2 then says which cards he would like to draw in order to make 1 using the cards already on the field. For example, since .3 and .5 are lying on the table, (all cards on the table are shared) he says “I hope I get 7 tenths, or five tenths, or 2 tenths! (considering he could add the .3 and the .5 to make .8, and he would need a .2). He draws and then either makes 1 or, if not, lays his card on the field and play goes back to Player #1. Combinations of more than two cards are possible and desirable, since the player with the most cards at the end of the game wins. We like this game because no one says “I hope I get POINT SEVEN”. They can see the 3 tenths in red, so they hope they get 7 tenths (of course, 70 hundredths would be fine, too!)
ALTERNATIVE GAME: “WAR” Each student gets their own stack of decimal cards, face down. At the same time, they both turn over one card. The largest decimal wins. Gradually one student’s pile gets bigger and bigger… it’s competitive, but it relies on luck, so students accept defeat when it happens!
ACTIVITY TWO – Number line
In 4 groups, students get 3 or 4 cards (mixed decimal and fraction cards).
The rules only allow one student at a time to tape up a card on their number line. Students take turns putting up one card, then they go to the second round, etc. They cannot grab someone else’s card.
They usually start with their most obvious cards (1/2, 1/3…) and work from there. And yes, they make mistakes, but they’re thinking. Sometimes we point to 2 or 3 cards (if one of those 3 is wrong) and ask if everyone agrees.
ACTIVITY THREE – Make 21
In groups of 2 or 3, students play the traditional game of Blackjack, but their goal is to make 21 tenths. Suppose a child receives 2 cards from the dealer (we let the dealer play, too), let’s say 5 tenths and 75 hundredths. They usually say “12 and a half tenths” and they usually ask for another card. (“hit me”…:) If they get 4 tenths (or 400 thousands), they have 16 and a half tenths, and they should hold, but usually they ask for another card and go over. You can play it online, too, at decimalsquares.com
We find we don’t even have to teach any “addition of decimals” rules. These games create a natural understanding of quantity.
Next post: Introducing Decimal Multiplication
