Can I intentionally play cards that cannot damage obstacles?
Yes you can play cards that will not do damage. The turn order is set up so that you have played all cards you will be playing before you actually apply damage.


Play Cards
Apply Damage
Take Damage
Draw &amp; Buy Cards
End Turn


From page 6 of the rules

This section states you can play cards even if there is no obstacle to use their damage against


  You play cards from your hand during your turn, one at a time.
  If your cards have any abilities other than damage on them,
  resolve those abilities when you play the card. Your starting
  decks don’t have any cards with non-damage abilities,
  though. When you play a card, you don’t put it into your discard
  yet. Instead, put it next to the obstacle that you want to
  damage with the card (you can damage obstacles facing any
  runner). You don’t place cards into your discard until after you
  apply damage. You are allowed to play cards even if there is
  no obstacle to use their damage against.  


This section says you can partially damage an obstacle on your turn


  If an obstacle has a damage track level that requires
  4 damage to clear, and you can apply only 3 damage
  to it, the damage on that level will reset at the end of your
  turn. The next runner will still need to deal 4 damage to it
  to clear it during their turn.