San Diego in October

Merino or synthetic. Cotton holds the sweat and you'll smell it by day two.

SimondSIMOND Women's Travel 500 Merino Wool T-Shirt$49.99Shop at Decathlon →SimondSimond Men's MT900 Seamless Merino Wool T-Shirt$45 was $74.99Shop at Decathlon →CoalatreeQuick Dry Short-Sleeve T-Shirt - Unisex$34.30 was $49Shop at Backcountry →KiprunKiprun Men's Run 500 Dry Running T-shirt$20 was $24.99Shop at Decathlon →

One pair for every two days, plus two.

SmartwoolEveryday Merino Bikini Underwear - Women's$34Shop at Backcountry →SmartwoolEveryday Merino Thong Underwear - Women's$34Shop at Backcountry →REI Co-opREI Co-op Women's Merino Hipster Underwear$29.95Shop at REI →SimondSimond Trek 500 Merino Wool Backpacking Underwear Women's$29.99Shop at Decathlon →

Trail runners dry overnight where boots take days, which is why most people prefer them on warm trails.

CraftCraft Women's Endurance Trail-Running Shoes$130 was $175Shop at REI →KiprunKiprun Kipsummit Max Women's Trail Running Shoes$150Shop at Decathlon →KiprunKiprun Kipride Women's Running Shoes$140Shop at Decathlon →ScarpaScarpa Ribelle Run Trail Running Shoe - Men's$116 was $179Shop at Backcountry →

Buy it used: REI Re/Supply sells gear like this for less. See used gear

Bring the pair you already walk ten miles in. Brand new shoes on day one is how the blisters start.

NnormalKjerag Shoe$109 was $140Shop at Backcountry →SaolaQuito Shoe - Men's$130Shop at Backcountry →SaolaEzo Shoe - Men's$83.96 was $120Shop at Backcountry →New Balance530 Shoe$60.47 was $110Shop at Backcountry →

Polarized kills glare, but the tag has to say 100% UV separately, and that's the part your eyes need.

CostaCosta Caleta Net 580G Polarized Sunglasses - Women's$174 was $248Shop at Backcountry →SmithSmith Cheetah Polarized Sunglasses - Women's$93 was $155Shop at Backcountry →ZealZeal Isabelle Polarized Sunglasses - Women's$67.03 was $149Shop at Backcountry →KnockaroundKnockaround Junipers Polarized Sunglasses - Women's$17.50 was $35Shop at Backcountry →

10,000mAh is about two phone charges, 5,000 barely gets you through one long day.

Top pickGoal ZeroGoal Zero Venture 75 Power Bank$120Shop at REI →BioLiteBioLite Charge 80 PD Power Bank$99.95Shop at REI →AnkerAnker MagGo Power Bank (10,000 mAh, Slim)$80Shop at REI →AnkerAnker Power Bank (30W, Fusion, Built-In USB-C Cable)$40Shop at REI →

REI members usually get 10% back on full-price gear. Membership is $30, once. How it works

65W or more charges a laptop. Anything smaller only keeps up with phones.

Peak DesignDual Slim Charger$64.95Shop at Peak Design →Peak DesignPeak Design 65W Dual Slim Charger$64.95Shop at REI →AnkerAnker Wall Charger (33W, 2-Port) with USB-C to USB-C Cable$25Shop at REI →AnkerAnker MagGo Wireless Charging Station (3-in-1, Foldable Pad) with Charger and USB-C Cable$100Shop at REI →

35 to 45 liters fits most cabins. Check your airline's size limit first. Most won't take much over 40L in the cabin.

Top pickMatadorMatador 45L Global Travel PackSee price15% off with CHRISANDSARAShop at Matador →MatadorMatador GloberiderSee price15% off with CHRISANDSARAShop at Matador →PacsafeGo Carry-On 44L Backpack$180Shop at Backcountry →Osprey PacksOsprey Packs Archeon 30L Backpack$132 was $265Shop at Backcountry →

Hard shell protects what's inside, soft shell squeezes into a tight bin, and either way check the size with wheels included.

CotopaxiCoraza Suitcase - Carry On$295Shop at Cotopaxi →SamsoniteHyperflex ∞ Carry-On Spinner$126 was $180Shop at Samsonite →SamsoniteVirtuosa Spinner$154 was $220Shop at Samsonite →ThuleThule Aion Carry On Spinner Nutria$350Shop at Backcountry →

15 to 25 liters.

CotopaxiElqui 24L Daypack$100Shop at Cotopaxi →CotopaxiConsuelo 22L Daypack$120Shop at Cotopaxi →MammutLithium 20L Daypack$135Shop at Backcountry →The North FaceIsabella 3.0 20L Daypack - Women's$66.75 was $89Shop at Backcountry →

Look for an IPX8 rating. Anything below that is splash proof, not waterproof.

Nite IzeNite Ize RunOff Waterproof Phone Pouch$34.95Shop at REI →ChromeChrome Large Phone Pouch$25Shop at REI →Peak DesignEveryday Case for iPhone 11 Pro$26.96 was $29.95Shop at Peak Design →Peak DesignEveryday Case for iPhone 11 Pro Max$26.96 was $29.95Shop at Peak Design →

Get locks marked TSA approved. Security can cut anything else off to search your bag.

MatadorMatador BetaLockSee price15% off with CHRISANDSARAShop at Matador →PacsafePacsafe ProSafe 620 TSA Accepted Luggage Locks$17.95Shop at REI →PacsafePacsafe ProSafe 1000 TSA Combination Luggage Lock$17.95Shop at REI →Samsonite3 Dial Travel Sentry Combination Lock$13Shop at Samsonite →

It runs on Find My, so this one's for iPhones.

AppleApple AirTag (2nd Gen, 4-Pack)$95.99Shop at B&H Photo →AppleApple AirTag (2nd Gen)$29Shop at B&H Photo →eufyeufy SmartTrack Link (2-Pack)$19.99Shop at B&H Photo →

Going in the water, buy the 80-minute one, since 40 minutes is barely one swim.

BadgerBadger SPF 50 Adventure Sport Mineral Sunscreen - 2.4 oz. Tin$18.95Shop at REI →Sun BumSun Bum Mineral SPF 50 Sunscreen Lotion$17.50Shop at REI →ThinksportThinksport Clear Zinc Active Face SPF 50 Sunscreen$15.95Shop at REI →Sun BumSun Bum Mineral SPF 50 Sunscreen Face Stick$14.50Shop at REI →

Picaridin at 20% works as well as DEET, and it won't melt your sunglasses or the plastic on your pack.

Ben'sBen's Eco Spray Insect Repellent - 30 Percent DEET$9.50Shop at REI →Adventure MedicalAdventure Medical Ben's 30 Deet Tick & Insect Repellent Pump$1.92 was $2.95Shop at Backcountry →Adventure MedicalBen's 30 Deet Tick & Insect Repellent$1.92 was $2.95Shop at Backcountry →Ben'sBen's Spray Pump Insect Repellent - 30 Percent DEET$5.95Shop at REI →