Puerto Rico... Nice! I just got back from a Caribbean cruise and spent a couple of days in Old San Juan.
The view from Cristobal Castle lookout west to the other end of town:
As for gear:
First of all, I recently purchased a circular polarizer for my kit and this probably had more impact on improving my photos in sunny places like the Caribbean than any other piece of gear I've ever acquired. No matter what you do in terms of lens choice, I highly recommend a circular polarizer. You can get a top of the line B+W Cir. Pol. filter at maxsaver.net for around $100.
For lens upgrades, you have a few choices:
Canon EFS 15-85 f3.5-5.6 IS USM: This lens is Canon's latest EFS lens design and it offers remarkably good quality and is priced accordingly. The build quality is solid and the range of focal lengths is very nice: 24-135 FF equivalent.
Canon EFS 17-55 f2.8 IS USM: This is considered by many as the ultimate walk around lens for Canon crop bodies. In fact, there are no other lenses in Canon's line up that offer IS and f2.8 in this walk-around focal range. Most consider it's optics to be "L" quality and it's priced accordingly. The f2.8 and IS makes this lens very flexible in low light.
Tamron 17-50 f2.8 VC: While comparable to the above Canon lens in features, it falls considerably short in sharpness. It's not as sharp as the non-VC version which is near legendary in terms of value. It's more affordable however.
Tamron 17-50 f2.8 NON-VC: This lens is probably the value king, offering great image quality, constant f2.8 at a very affordable price point. However, it lacks IS which can be a real compromise for shooting static subject early or late in the day without a tripod (or indoors without a flash).