Welcome aboard, Veronica!
Gratuities can certainly still be paid in cash should you prefer that. Simply stop by
Guest Services on Deck 3, Midship and let the Cast Members there know you'd like to pay gratuities in cash. If your credit card has already been charged, I believe they can credit your account, but do chat with them about the specifics.
Another option is to do as you mentioned, and purchase Disney Gift Cards or use your Disney Visa Rewards Card to credit your stateroom account. Guests can call Disney Cruise Line prior to setting sail to make payments to their stateroom account or stop by Guest Services to do so onboard. My family does this every time we sail. In fact, there have been a couple of times when I've not spent as much onboard as we've pre-credited with gift cards to our account. If that happens to you too, just stop by Guest Services at the end of your sailing and they will credit your Disney Gift Card with the remaining overage. Just be sure to save the same gift cards you put on the account to begin with in case you need a credit.
Lounge hours vary each day (especially in port), and you'll want to consult your Personal Navigator (the onboard schedule) each day for exact times. However, typically the hours on sea days are:
Bon Voyage: 10 a.m. - 11 p.m.
Cove Cafe: 10:30 a.m. - 11 p.m.
Currents: 10 a.m. - 11 p.m.
Meridian: 10 a.m. - midnight
O'Gills: noon - midnight
Ooh La La: 5:30 p.m. - midnight
Skyline: 5:30 p.m. - 1 a.m.
The Tube: 9:30 p.m. - 2 a.m.
Vista Cafe: 7 a.m. - 11 p.m.
Waves: 10 a.m. - 6 p.m.
Have fun planning the rest of your cruise and let me know if I can help with anything else, Veronica!