You definitely want to make sure you dress for the weather. Additionally, when you consider your location and activities, you'll be more prepared when it is time to pack. Outdoor types tend to hike, walk or bike on their vacation, so you'll want to pack hiking shorts or pants (or even better the type that converts), walking/hiking shoes, good socks that won't give you blisters, and breathable shirts. If you're planning to bike, bring an outfit suitable for that as well. Considering layering will also be helpful.
Keeping in mind gender differences, men and women will need different toiletries. A few things are universal though, such as toothbrushes and toothpaste, soap, shaving supplies, shampoo and if you can spare the room, bring your own toilet paper. You never know when you'll need it especially with the lifestyle you lead on your vacations. Also, don't forget the sunscreen. No matter where you go and what the weather is, you can't go wrong by wearing the SPF.
Expect the unexpected. Keep a flashlight and batteries handy. If you're going to camp, this is especially important, but flashlights can also be useful for night hikes as well.
Keep your keys with you on your trip unless you have a trustworthy neighbor who can housesit for you. This also goes for your car.
As an active person whether you're staying in a resort or you're simply planning to camp, don't forget your gear. This may include snorkeling gear, fishing poles, backpacking equipment like backpacks and poles or even a simple Frisbee. It can sometimes be a hassle renting such things from resorts especially when you know you have it sitting at home. If you plan your itinerary ahead of time, you'll know better what to bring. Running shoes should also be packed because whether you're going for a morning jog or simply walking around the town, you know you'll be upset if you forget those.