The daytime action in Rio revolves around Beach Cafes that line the Copacabana Beach. Your other daytime options include agency girls and termas (but termas don't really get going until 5pm or so).

The top 2 are Meia Pataca and Mab's

One thing to be careful about in the Beach Cafes is be very careful with girls that have been eating/drinking when you join them. A popular scam especially if the girl figures out that you have decided not to take her back to your hotel is for the girl to go to the bathroom and then disappear and leave you with her tab. That is not problem if she just had 1 beer but if she ordered up a bunch of food then vanishes, it sucks.

Lots of girls hit the beach in the late afternoon, enjoy the beach, then head for the cafe after the beach (maybe 4-5pm). So you can find the girls both in the cafes and/or across the street depending on the time of day.