Downtime Activities

Things that can happen between adventures (p127 DMG):

Building a Stronghold

A stronghold gives you and your allies a place to bunk for the night and hold treasures.

Carousing

Drinking, whoring, and boozing might make you money or cost you money.

Crafting a Magic Item

Magic items can be crafted if you have the formula to do so.

Gaining Renown with an Organization

A faction or group likes you more if you work for them.

Learning a Language

Languages can be learned if you have a tome (or equivalent) to do so.

Maintaining a Lifestyle

Sitting on your butt costs money, get a job.

Performing Sacred Rights

Working at a temple gets you inspiration.

Running a Business

A business must be maintained or it goes into debt.

Selling Magic Items

Magic items can't be sold to just anyone.

Sowing Rumors

Can change a settlement's standing with you from Hostile > Indifferent > Friendly.