we need to try bir gunum paragraph in present tense:

Simple Present:

We need to try = Denemeliyiz. actually i used  -meli (should) for this one.
Exact translation: Denememiz gereklidir or gerekmektedir.   it sounds bit formal.

Although the sentence is gramatically correct in Turkish.
Daily speech you should use the phrase like this : 
´´ Arkadaşlar !/Kanka! Bu faaliyeti denemeliyiz.´´  translation ´´Friends!/Buddy We shoud try this activity.´´

One day, We need to try this activity = Bu faaliyeti bir gün denememiz gereklidir.  -formal way to say.  if you use this pharase as it is, it sounds robotic or formal. 

Daily speech- Bu faaliyeti bir gün yapalım.  One day we should try this one.
(Ok Sure- Peki tamam olur.)


ıF you suggest to  the activity now, then you should use this phrases like this -- 
Hey ! bunu deneyelim mi ? (Can we try this one)

or 

Arkadaşlar bunu şimdi deniyebilir miyiz ? (Exact translation Friends, Can we try/try out this activity right now ?)