There is no way that you won’t look like a foreign tourist. Your clothes, footwear, walk, haircut, mannerisms….all of those are instantly recognizable. Plus that camera you'll be toting. You can try dressing as some Brazilians dress. For young guys, especially in seaside locations (even Rio), this might be board shorts and a T-shirt (no writing or only certain brand names—Billabong, for example) with sneakers (with no-show socks/peds only) or flip-flops (Havaiianas brand are sold on every corner in Rio).

As far as sitting or not sitting on a towel, you do not even bring a towel to the beach. Women use “cangas” (sarongs) and you may use a corner of your partner’s (if you have a partner and she has a canga) if needed to dry your face, but you will usually dry off from the heat.
At the beach, there are barracas (stands) that rent chairs and umbrellas, and the guy in charge will also supply any food and drinks you want, although there are usually also passing vendors of other items. This is the setup in Rio and at the most popular beach in Salvador (Porto da Barra). Mar Ipanema may have its own items/barraca for its own guests, as some Rio hotels do.
At other beaches, the barraca has groups of plastic tables with umbrellas and chairs already set up and you choose which table to sit at and buy your food and drinks from the barraca to which they belong. Some are “fancier”, have lounge chairs for example, and may charge more for food and drink.

As far as safety, try to look purposeful (i.e. not clueless) when walking, read the many online safety tips (no bling, keep out of deserted places, keep the camera put away unless actually taking a photo, etc), don't let yourself get too impaired by fatigue or alcohol to pay attention, and try to make all this look as effortless as the Brazilians do. ;-)