There are data loggers made for the food shipping industry that log temperature and humidity and are relatively inexpensive and very easy to use. An overview of the products is here: http://bit.ly/UdVImh . They are the size of a small stack of dimes.
Citizen science participants in NestWatch used them for several years to gather temperature data from bird nests and nest boxes. Participants would program loggers, deploy them, and then download the data and email data files to researchers.
The Thermochron iButton measures only temperature and there are many styles http://bit.ly/SPIISn. The least expensive is only $3.80/logger, but it won't keep real time records; the cheapest that keeps time-temperature records is $14, if you buy bulk of 1000 loggers.
The Hydrochron iButton measures temperature and humidity at a bulk price of $55/logger if you order 1000 loggers. Here are the specs: http://bit.ly/UgYvrI
Step 1: Delivering the recorded data
If participants are going to download the data and submit it to a website or via email, then each would also need a USB adapter to download the data from the iButton to a computer http://bit.ly/T6MBSI (cost $14). But the loggers are tiny and easy to mail back to researchers for downloading at the end of the sampling period.
The data loggers are re-usable. Depending on the sampling period needed at each site, it could be easy for participants to take turns using data loggers, sharing through shipping in the mail.