I like brittle stars. The minis and some of the larger species are good additions to a tank. They're unobtrusive, mild-mannered, and always on the job. The minis tend to be suspension feeders capturing particles out of the water - you can tell because the arms are waving up in the water. The larger ones sold in the trade tend to be deposit feeders but there are some carnivores to be avoided such as the green brittle star Ophiarachna incrassata. It really will capture & eat small fish as well as snails, worms, shrimp, and pretty much anything it can catch. Ophiarachna is fun to watch but not when it's devouring some expensive blenny.
People have mixed opinions about urchins. Again, you have to do some research & get the right kind for your problem, whether algal growth or debris build-up, and avoid the carnivores that will decimate the CUC in your sand bed or rock work. Even the best ones can knock over rocks & corals (except for rock urchins which stay in one spot) and some species, while good CUC, pick up corals & rocks as camouflage.