What to do with this layover

A gap between flights that's too long to sit in and too short to relax about.

I have a layover and need to decide whether to leave the airport.

Work through this in order:
1. Subtract the real overheads from my total gap: immigration both ways, security again, bag situation, the journey each way, and a safety margin. Tell me the usable time that is left.
2. Give me a clear verdict: leave, or stay. If it is marginal, say what would tip it.
3. If leaving is sensible, give me one plan only, with a hard turnaround time by which I must be heading back.
4. If staying, tell me how to spend the time inside well: where to actually sit, eat and wash.

Do not state visa or transit rules as fact. Tell me exactly what to check, and whose website to check it on. Treat any transit time you give as an estimate, not a promise.

Airport, arrival and departure times:
Passport and whether bags are checked through:

During and after the trip. Layovers, days that fall through, what is worth doing nearby, and writing it down afterwards.