Hi Natalia,
Yes non-alcoholic, non-specialty drinks are included on the Disney Dining Plan (DDP) at both counter and table service restaurants. You normally have a choice of different sodas, lemonade, milk and water, these do vary between restaurants and some even allow you to have milk shakes or smoothie's. I can definitely recommend the Oreo milk-shake at Sci Fi in Disney's Hollywood Studios, it's delicious! You can find the DDP brochure with details
HERE.
Tips are not included on the DDP so you will need to pay for these yourself, the restaurant does make it very easy for you to do this as you can pay with cash, credit card or charge it to your room.
Refillable mugs are great, my family always buys them because we normally vacation for 14 days so as you can imagine we drink a lot in that time. The drinks that are normally available are tea, coffee, soda, fruit punch and lemonade and the price at the moment for the mug is approximately $13 plus tax. I'm not sure how many days your trip is for but with the average price of a soda being about $2 a go the mug would very easily pay for it self quickly.
Hope that helps and you have a fantastic trip, oh don't forget to book your dining reservations on 407-939-3463 up to 90 days in advance.
Jo