I am a litlle confused! Does the frankfurt day pass include Offenbach and airport?
You can buy "pure" RMV public transport day tickets. A day ticket from Offenbach to Frankfurt is EUR 9,55 - less than 2 single trip tickets EUR 4,90 each). And is also valid to Frankfurt Airport.
An alternative is the RheinMainCard. EUR 22 for 2 days. Covers the entire RMV area. Gives also discounts, but for far less places in Frankfurt than the FrankfurtCard. In Frankfurt for the Zoo, the PalmenGarten and the Primus boats. But e.g. a 50% discount of the German Leather Museum in Offenbach (the most interesting sight of Offenbach).