I understand from another question that there will be an API to allow the public to be able to contribute to the experiment sometime in December.
Could you advise of a particular type of sensor that would be useful for this?
Dominik Schiener said in one of his latest tweets that they are currently "working with selected hardware providers".
8/ With all of this, we hope to launch a production ready marketplace in 2018, and are already now working with selected hardware providers on integrating MAM in their SDK's.— Dominik Schiener (@DomSchiener) 28. November 2017
In another recent tweet he stated, that, if you live in Berlin, you can already sell your data if you have a LoRa sensor:
5/ In Berlin, we will work together with a partner on setting up a LoRaWAN network coupled to the marketplace. So if you're in Berlin and have a LoRa sensor: great, you can now sell your data!— Dominik Schiener (@DomSchiener) 28. November 2017
For more about LoRaWAN Sensors you can visit their official website. They already offer a wide variety of different sensors but if I understand the first tweet correctly there will be more to come in the near future.
First of all you have to own the data and the data has to fulfill certain legal requirements.
No real world payments or other real world financial consequences will result from this experiment. All data being contributed to this proof of concept is either non-sensitive data of which the participants are the authorized owners and/or is publicly available data which the participants may freely choose to share. Participation in the IOTA Data Marketplace takes place on a voluntary, non-contractual basis. Participants may choose to discontinue their participation at any time."
Source: Disclaimer on https://data.iota.org/ (Emphasis mine)
Since the same disclaimer also states that no real world financial consequences will result from this (since it runs on the IOTA testnet) all other considerations are moot for the experiment. That being said just pick the cheapest sensor or data generator you can think of (probably a RNG on a device you already have) and connect it to the experiment in December to find out how the technology works.
When the actual market is launched you'll have the usual things to consider when you want to provide any sort of service or product. It has to provide some sort of value to the buyer. Most likely the first legal requirement still applies or will get even more restrictive.