Beyond this weekend’s Common Ground Country Fair in Unity, there’s lots more to do around eastern Maine. Penobscot Theatre Co. opens its 2024-2025 season with the comedic play “Birthday Candles,” which runs through Oct. 6 at the Bangor Opera House. On Saturday, take your mermaid-obsessed kids to the Grand Theatre in Ellsworth for a 7 p.m. singalong to the classic Disney movie “The Little Mermaid.” On Sunday, the Bangor Symphony Orchestra opens its season at 3 p.m. at the Collins Center for the Arts Orono, and there’s the final Bangor Waterfront Night Market of the season, set for 4 to 9 p.m. along Bangor’s Penobscot River waterfront.