A big thank you composed as a poem or a written piece. The words alone can speak for themselves and the effort put into the piece. It's easy to get the flowers and the gifts offered commercially, but somehting offered from the heart is priceless, lasting, memorable and so, so endearing.
If you're gifted musically, try composing a song and if you're not a good singer get someone to sing it. Your parent will love it and the song will go straight to the heart, never to be forgotten.
It doesn't cost much to do these things - just a few hours and the memories will last a lifetime.