This type of exercise bike allows you to sneak in some extra workout during those times you need to be occupied with other things as well, like work or conversation. There are plenty of choices out there, from very affordable ones with no frill (and no pedal resistance) to ones with inbuilt flywheels and timers.
They all share a common trait though. They are practical and small so they are easy to carry on trips and during travels. Usually, they are an ideal solution for people with ankle and joint problems in the lower extremities.
If you require your mini exercise bike to offer some resistance, pick one that has a fly-wheel. This will allow you to put in serious practice, as it offers pedal resistance, unlike mini exercise bikes that do not have a fly-wheel.
Keep in mind that, while mini exercise bikes do not offer a full workout, they are a great way to compliment your regular daily routine. They are a great addition to your home gym, and are very affordable, so deciding to buy one will be a cheap, yet worth-while, investment.