honestly your last idea about Gohan needing Piccolo to step in for a father-son thing and everything that spirals off of that sounds like an amazing idea that'll be pretty funny, but i'd say let's save that for later.

So here are my two ideas for now:
1) Gohan and Piccolo go fishing. This would probably end up more feelsy, since they'd likely end up reminiscing about Goku and the past.
2) Gohan comes to Piccolo looking for advice about girls. He's the former guardian of Earth, surely he knows all about women right??? this would definitely end up more comical for obvious reasons.