I second Hydroguy's suggestion. You'll be there in the summer. It will be crowded. For our family, the #1 thing we do to ensure a good & enjoyable, yet not too cranky or exhausting, time is to stay within walking distance.
#2 thing to do:
Take a break in the afternoon at the hotel. This is why you stay w/in walking distance. For us, "walking distance" is ~10 min of a walk. If it's a 20 min walk, we'd take ART. But taking ART to/from the parks makes taking a break at the hotel more challenging.
#3:
If your budget allows for it, get park hopper tickets. This gives you flexibility when it's crowded. Park hopping takes <5 min from 1 park gate to the other.
#4:
Buy your tickets ahead of time so you get a Magic Morning entry to DL on Tues, Thurs, or Sat. And use the MM entry early in your trip. MM/EMH at DL in the summer is usually 7-8 am. It's totally worth it to get up that early. Then take a break at your hotel (see #2).
#5:
Have each person in your group pick THE #1 thing that they want to do on the trip and try to make it happen on the first day. So if their #1 thing is down or fireworks are cancelled, you have the rest of your trip to still make it happen.
All of the above I got from reading Hydroguy's tips compilation threads. Speaking from personal experience following the above suggestions a few times now, it totally makes a difference.
Also, staying "on site" is not as much of a "must do" at DL compared to WDW.