Go down to the local bend in the river and meet the locals. The moms come down in the afternoons and swim with their children. The children are extra friendly and smile easily. Water is very refreshing.
If you happen to be in El Fuerte on Nov 1 & 2 (Day of the Death/All Saints Day), you might want to check out the Pantheon (local cemetery).
My American travel buddy had witnessed D of D celebrations in another part of Mexico and was so thrilled by the experience she talked me into going to the cemetery in El Fuerte (as well as the one in Batopilas). I'm glad we did because we got quite a bit of insight into how people celebrated life and after life. In El Fuerte, the celebration was on a smaller scale.
The gravesites were spruced up for the occasion and decorated with flowers and wreaths. It is similar to the Chinese "ching ming" in April.
We ended up visiting the Pantheon twice that day. Evening is the best time to visit the cemetery because the atmosphere is quite festive with lots of lit candles, and street vendors selling snacks & drinks outside the gates.
Museo El Fuerte: the outside is more impressive than the inside. But for 5 pesos, we just wanted to check it out anyway.
From the top of the museum, which looks like a fort, you can see the river and the town.
El Fuerte has the typical Mexican town centre which consists of the zocalo with an adjacent civic building and church.