Rainbows are tricky and generally more advanced, they are susceptible to humidity related issues like respiratory infections and shedding problems. They are great if you can properly care for them but be prepared to pend a little extra money and time caring for them.
I would suggest the red tail boas, sand boas, dumerils boas, or (my favorite) the madagascan ground boas. All of these have very different requirements and looks but are generally easy to care for. Red tails will get the largest and are usually quite easy to tame, Dumerils don't get quite so large but are also easy to tame. Sand boas and ground boas stay small but one is from the desert and the other from a forest- it is all up to your personal choice.