A good rule of thumb to start out with is to pack only clothing that you are absolutely sure you will wear. Doing this will save you time packing as well as suitcase space. The downside is that you might be left needing to buy clothing to suit a particular need you didn't plan for. If you are planning on being outdoors a lot, it certainly makes sense to plan for at least two outfits for each day: one for hiking or biking and one for hitting the town. Check the forecast for as long as you plan to be there, and alter any packing plans according to weather needs.
Every vacationer needs to pack toiletries, but if you plan to work up a stink by climbing rocks or snowboarding, then make doubly sure you've got all you need to look good at night. Pack these items in a separate bag or inside a baggy you keep in your large suitcase. Make sure you have shampoo and conditioner, comb or brush, hair care products, dental floss, toothbrush, toothpaste, deodorant, contact lens supplies, if necessary, feminine hygiene necessities, and perfume or cologne. If the trip is going to be short, try to find your favorite products in travel sizes.
Certain items are vital for those who plan to hit the trails. Get yourself the most up-to-date maps, and don't forget your handheld GPS device, if you have one. Pack a flashlight and plenty of batteries. Add some earplugs, chewing gum for high altitudes, bug repellent and sunscreen. Don't forget the camera and either film or media storage cards.
For the outdoor vacationer, packing the first-aid kit is more important than it is for the resort traveler. As a result, your first-aid kit should be much more comprehensive and considered absolutely vital. A solid first-aid kit should include whichever pain reliever works best for you, bandages, a snakebite kit, Benadryl, water purification tablets, a lighter, whistle, an epinephrine syringe pre-filled for those who are allergic to bee stings, anti-itching cream, antibiotic ointment and a fully charged cell phone.