Hi Chris,
Converting cash to Disney Gift Cards is a good way to avoid carrying around lots of bills. You can also arrange to allow charging privileges on your Key to the World card (room key), so you can avoid carrying lots of cash that way as well.
You should know that restaurant gratuities are not included on the Disney Dining Plan. You should budget to spend about 18 of the cost of any table service dining you do. You'll also want to keep a bit of cash on hand for things like tipping bellmen or taking a taxi.
As for souvenirs, the sky is really the limit depending on what you're interested in. Disney sells everything from $2 themed pens, to diamond Mickey Mouse watches that cost several thousand dollars. In the middle, you can expect to spend about $15-30 for a tee shirt, $45-60 for a sweatshirt, $10-15 for Mickey ear hats, and about $10 for a trading pin.
You can take a look on DisneyStore.com and search in the
Parks Merchandise section for an approximate idea of what other souvenirs might cost.
For more information on tipping at Walt Disney World, you may want to head over to the independent website blog.touringplans.com. Search there for an article called "Frequently Asked Questions about Tipping at Walt Disney World." And for idea about the pricing for other vacation add-ons search for the article called "Doing Everything at Disney World: What Would it Cost?"
Good luck with your planning. Please let us know if you have additional questions.