Make sure to include the following items in your budget, as well as anything else you think you will spend money on:

• Roundtrip airfare for three to your destination (Is the price the same for everyone in your group?)

• Hotel costs for 6 or 7 nights (Will it be the same each night? Will you stay in more than one hotel? Make sure you price hotels for three people, one of whom is under 18)

• Approximate cost of breakfast for 7 days (Is breakfast included or an option at any of the hotels?)

• Approximate cost of 7 dinners, detailed by cost per night

• Approximate cost of lunch for 7 days, detailed at much as possible (picnic with sandwiches, etc)
  

• Cost of each attraction you plan to visit (Itemized one attraction per line in your spreadsheet.)

• Local transportation cost for the week (whether you plan to stay in one city and travel by bus or subway, or whether you plan to take the train to another city)

• Souvenirs (itemize things you might buy, and price them as best you can)

Notes:
1) You will need to determine the price per person, and then calculate the total for all three of you.

2) Keep in mind that you will probably cost less than your father, and your grandmother will probably also qualify for some sort of discount.