Here's a few:
1. The Vale of Rheidol line passes Aberystwyth Cricket Club and there's a good view of the whole ground.
2. Ludlow Racecourse is next to the line with a good view.
3. You can still have a view of part of the ground from a tram passing Old Trafford, but I doubt that it's very good since they built the new stands.
4. You can see the outside of the Liberty Stadium, Swansea (Swansea City and the Ospreys), from the line into Swansea, but very little of the ground.
5. The line to West Wales passes Gowerton Cricket and Rugby Club just after leaving the station, and there's a good view.
I'm sure that I've seen a few more, but can't remember them at the moment.