It's hard to describe it as a Treptower itinerary or a Friedrichshain one, anyway I guess that the best was of doing it is absolutely by bike or at least enjoying half a day around the area.
Full map: http://goo.gl/maps/QV61I
You can start from Rummelsburg of from Treptower Park, depending if you like to stay in the nature in the beginning or relaxing in the end. From the Treptower Park side you can enjoy the park and go visiting the Sowjetisches Ehrenmal - Soviet War Memorial (7), the soviet cemetery which will surround you in a static climate, like if the time was never passed in the last 50 years. Then you can have a visit to the Insel der Jugend - Isle of Youth (8)with its typical bridge and a nice bar to have a coffee or something to eat or to rent a kayak in summer time.
Going back more on the Kreuzberg side on Sunday you can spend some time snooping around the Arena Indoor Market - Flohmarkt (6) where you can find some funny oddities and rare things. When you're ready to go and you're not too full of gifts, you can walk and on the Schlesische Straße there's a big "Urban Void" where you can watch a really big graffiti by Blu. At corner with the Oberbaumbrücke (4), the two towers famous orange bricks bridge, symbol of Friedrichshain, it you can have a look to the Bonjour Tristesse (5) building. The building itself, built by the architect Alvaro Siza, is not very nice, made of raw gray concrete, but the writing on the top is what make this building special and it deals well with the general atmosphere of this area. Going as you want to cross the bridge, on your right on the last two buildings you'll find other two graffiti by Blu.
It's now time to cross the bridge, then the S-Bahn bridge and finally turn right. It's a quite long way, but the great RAW Tempel (3) area is waiting for you. The name derives from Reichsbahnausbesserungswerk, because it was an area belonging to the Railways. Now it's full of activities from a skateboard hall to a climbing school and a lot of places to go dancing or listening music by night and on sunday you'll find here a very interesting flea market (but just in spring/summer season).
You can walk into the friendly district of F'hain for a bit more, leading Ostkreuz where you'll find its typical Wasserturm (the Water tower). From here, crossing the station and going to Rummelsburg, you can have a look at this very quite area, almost renovated and to the Knabenhäuser - Boys' houses (1), an orphanage on the river Sprea.