drop-in 
adjective
1Visited on an informal basis without booking or appointments: a drop-in disco...
- It is an informal group that meets once a week on a drop-in basis.
- Most of the time it is moored up as an informal drop-in restaurant for everything from a cyclist's brunch of French toast, bacon and maple syrup or pan-fried mushrooms on toast to a well-earned coffee and cake at the end of a walk.
- To try to halt the explosion of cases, health bosses are organising drop-in sessions, school visits and extra GP surgeries to vaccinate teenagers and young people against the virus.
2(Of an object such as a chair seat) designed to drop into position.It's easy to re-cover a drop-in seat if you have some leftover upholstery fabric....
- The drop in seat telescopes to the right width for your canoe and clamps onto the gunwales with clamps.