Latin adjectives usually follow the noun they describe; they also agree in gender, number and case with the noun. If the word "ideas" is feminine and nominative plural, then the adjective "bright" must also be in its feminine, nominative plural form.
Latin has many words meaning "idea": species, forma, imago, notitia, opinio, sententia and several more. Cogitatio means "thinking, meditation, thought, intention, plan or opinion" - so much the same thing as idea.
You wanted the plural, which for cogitatio would be cogitationes.
Bright (in the sense of clever, smart) is argutus or sollers - sollers is perhaps most appropriate in this case. Illustris means something that is literally shining or glowing, rather than figuratively.
So: Cogitationes sollertes - Bright ideas.
"Patres nostri et nos hanc insulam in brevi edomuimus in brevi nostris subdidimus legibus, nostris obsequiis mancipavimus" - Walter Espec 1138