I just got back from my first cruise, 3 days on the Magic. I also didn't want to spend too much (am going to WDW in 3 weeks), & found it was easily possible to keep spending low.
I don't drink alcohol, am not that into spa treatments and am not addicted to buying trinkets, so I spent relatively little money during the trip. My big splurge was bingo, because I really enjoy the game (I spent $175, & won $60.) Lucky for my wallet there was no casino aboard.
Our only port was
Castaway Cay, and it was so wonderful that no paid excursions were necessary (or even desired- the whole point is to relax & go at your own pace). I paid about $20 total for my son and I to rent bikes.
I had already prepaid standard gratuities before the trip, & added cash to my dining server's & stateroom hostesses envelopes, so my total bill end of cruise was about $200- it was just the bingo & bike rental.
I had considered pre-purchasing a photo package but didn't, & I'm glad I didn't. There was ALWAYS a CM happy to take the character photo with my own camera, and they were just fine. The professional photographer photos weren't uniformly great, anyway (one staged picture they took accidentally had me talking- hello?)
One more tip to avoid a big bill: make sure you keep your devices in Airplaine Mode, stay off the Internet & disable automatic downloads, etc. Very glad I got that tip in advance (thanks, Dis!) Better yet, just turn your devices off for a few days & enjoy a holiday from electronics- it'll help you soak up the beauty of your cruise a bit. Disney pushes their Personal Navigator app, but I liked the paper copy better.