Play games: dice, board games, card games. So many of them include counting and "adding on". Or an online game like this one.
Play store, where you pretend to buy things.
Draw a circle numbered 1 to 10, and draw lines between the pairs that add to 10. Then decorate it. If she likes this, do a circle numbered 1 to 12, and connect the pairs that add to 12. ...
Typical strategies include:
Memorizing sums to 10, e.g. $7 + 3 = 10$, then using that equation to determine $6 + 3 = \_\_$ or $7+4=\_\_$
Memorizing the doubles, e.g. $7 + 7 = 14$, then using that to determine $8+7=\_\_$
Decomposing/Recomposing. e.g. $9+8=10+7=17$
Using single digit facts to determine sums within 20. e.g. Since $3+6=9$, then we can easily ...