Bless your heart--I feel your pain. My initial thought while reading your request was, "Whaddya mean?? Wear shorts and T-shirt!" Ha ha ha--I get the chick factor, though. You want to look effortlessly cute and comfortable, and you want it to weather all storms that the day might entail. Previous poster is right on.
Let me tell you what I would wear starting at the top--ultra-conditioned ponytail (to deal with sun/heat) with a cap/hat. Facial moisturizer and tint, or spot concealer on face. (I rarely smear foundation on my whole face. I just blot it on spots and blend, so my face can breathe and I don't look too made up.) A little bronzer on my cheeks for "casual color". Water-proof mascara. Sun-block all over. Sunglasses that you don't have to take off or that you can just push up on your head when needed. My favorite lip gloss. (I always feel good when my lips look great. Your self-esteem might need an extra shot walking around with these girlies.) I prefer to be sleeveless, so a flattering tank top (maybe doubled) with a very casual cargo skirt or very, very loose shorts. (I like for my body parts to breathe!) I don't do well with closed-in shoes, but cute little shoes are a no-no, so I would wear my Fit-flops. The soles are great for walking and standing (I use them in my kitchen.), and I can show off my pedicure. Invest in some athletic flip-flops, something easy on the foot. I keep my accessories simple--small earrings, wedding rings, link bracelet, simple ring on right hand, small chain with pendant around my neck. These are things that I always wear. I also wear body jewelry--rings on toes, bracelets on ankles, chain on hips. Maybe you'll feel better about it if you can dress it up with a toe ring and pedicure. (If you are considering getting your first toe ring, get it early and walk around for a few days, so you can be comfortable walking with it.) Sometimes it just takes a little kick (nobody has to know that it's your kick) to make you feel good in certain environments. Wear a swimsuit or cotton underwear. If you don't have to wear a bra, wear one of those cotton tops with straps and built-in bras. (I get mine at Victoria's Secret, just to let you know what I'm talking about.)
I would also strongly suggest that you start your outing feeling as rested as possible. You'll look and feel beautiful if you feel rested.