When considering short term sign solutions, you should consider the environment the sign is to be used in, how long the sign needs to be in place and any other practical considerations.
If you need a temporary sign somewhere with a metal backing, especially if the sign needs to be removed or changed frequently, then magnetic signs are a good solution. Magnetic signs are easily affixed to metal surfaces, including car and van bodywork, and are usually very flexible, allowing them to be rolled up for storage when not in use. Magnetic signs have a typical lifespan of around 5 years in normal use, and are well priced against other options for use over several years but less cost effective if only in use for short periods of time.
For very temporary signs in good weather conditions or indoors, consider correx signs. Correx signs are made of a fluted plastic called polypropylene, which is like a plastic cardboard. Correx signs are cheap and easy to manufacture, but not very durable and unsuitable for use in high winds. Correx is actually a specific brand of corrugated plastic, but is so popular in Europe that the brand is now a generic trademark for all corrugated plastic. Correx is also known as twinwall plastic, and is fully recycleable. It is typically made in sheets 4mm thick, and it is unusual to find other thichnesses.
For a slightly longer term solution, foamex signs can be used. Foamex is a rigid plastic made of compressed PVC, and is waterproof and suitable for short term use both outdoors and indoors. It can be used to create a stand-alone sign, or can be backed onto plywood for a more durable sign. Unlike Correx it comes in a variety of thicknesses, typically up to 10mm. Foamex signs can be easily drilled, screwed or even nailed to mounting surfaces, making these very versatile but still affordable signage options.
For longer term use, acrylic signs or aluminum or dibond signs are much more durable, but more costly than correx signs or foamex signs. These are typically made to be permanently mounted for long term use. Solid aluminum signs are typically mounted on poles, and this is the preferred material for motorway and road signs. Dibond (aluminum composite) signs are also strong, but are typically larger signs mounted against walls or similar solid flat surfaces. Acrylic signs have a good polish and finish, and can appear very close to glass, making these the sign of choice for professional applications, and are typically smaller signs with company logo and branding at entry points. These signs are very hard wearing and durable, but too expensive for temporary use in most cases.
Correx signs are ideal for single uses for short term events, such as parking direction signs for a large event or route markings for a fun run. Many companies which manufacture correx signs will have a minimum batch run, as these are rarely used for a single sign, so that may be a factor in determining the choice of material. Foamex signs are more suitable for larger signs and for longer term use, such as roadside advertising for a business.