You have many many many....many choices.
Turtles, i'd say no. Plecos ("suckerfish") are more trouble than they are worth IMO.
What do you want to see in your tank? We can't tell you what you think looks good. Color? Activity? Etc..? I'd go ahead and say go African cichlids as they present more color and activity, at least in my opinon. As for what kind of africans well....you have your choice between Lake Malawi, Lake Tang., or Lake victoria. The normal "Mbuna" from Lake Malawi are pretty good starters and they come in a lot of colors.