Portable toilets are very convenient, but they do have a rather questionable reputation because sometimes they're not taken care of that well. The popular image of a smelly, dirty portable toilet is not really what most of these look like. Yes, it's possible for someone not to take care of the toilets at their worksite, for example, but most of the time, these structures are kept clean. One issue that can still occur even in clean toilets, though, is odor. There are three things you can do to control the inevitable odor, however, so don't let the idea of a smell make you wary of these toilets.
Yes, you can get exhaust fans in these toilets. The fans are small, but they work well and remove odors quickly. If the fan is connected to a switch other than the one for the light, you may want to post a sign asking people to flip both switches so the fan runs while they're in there. It's best to look for a fan that is connected to the light switch.
You have got to have the toilet serviced and the tank emptied regularly. If the toilet will be at a busy site, like a carnival, arrange for someone to check the tank and empty it every couple of hours. The toilet manufacturer should be able to give you a rough idea of how long the toilet can go without being emptied, but to be on the safe side, service the toilet more frequently. This way, anything that can leave an odor is gone fairly soon.
There are special deodorizer sprays made just for portable toilets. Have these installed if they do not already come with the toilet (if you're renting the toilets, look for models that already have these dispensers). While not everyone will like the deodorizer smell, it is definitely not as offensive as the odors it would be covering up. These deodorizers are available in conventional and eco-friendly formulations.
To get a better idea of which fans and deodorizer dispensers are available, or for how long the toilet can go before it needs service, talk to a sales or rental company, like Mountain Waste & Recycling. They will have the specifications for each model and accessory, and they may even offer service contracts so that you do not have to arrange for a third party to empty the tank.