You can cross the Hawthorne Bridge and either go directly to Mt. Tabor or (better, yet) ride south on the Springwater Corridor to Sellwood itself. The Sellwood Bridge is NOT a pleasant bike ride, whether on the narrow sidewalk or on the traffic lane. I really recommend, however, that you ride north along the river wall on the west side of the river up to the Steel Bridge, cross on the bike/ped level, and then take the Eastside Esplanade, which morphs into the Springwater Corridor. You would get some great views of the city going and coming. And there's nothing wrong with coming back the same way. This whole little jaunt may take an hour and a half, plus extra time if you want to stop and enjoy the river views and watch birds. There are nifty little restaurants and coffee shops in Sellwood, if you want to ride up the hill on Spokane Street to 13th and mosey around.
If a first-time Portland visitor has time to take only one bike ride, this is the ride I'd recommend. Believe me, you won't be disappointed, even if it's "misting" a little