Bryn Meurig Bach is a treehouse in Glantwymyn, near Machynlleth. It sleeps five. One bedroom, one bathroom. It's got a 3 Star rating, which is about what you'd expect for something like this. It's clean and works.
The main bed is a double, and there are three bunk beds for kids. The living space is all one room under the roof. There's a wood burner for heat. The kitchen is small but has what you need. The shower is outside, under the main structure. The toilet is a compost toilet, also outside on its own platform. You get to it via a rope bridge.
It's in the trees, so you'll see birds and squirrels. There's a stream nearby. It's quiet.
Machynlleth is about five miles away. It's got shops, pubs, and a market on Wednesdays. For walking and climbing, you're at the edge of Snowdonia National Park. The coast at Aberdyfi is about a 30 minute drive. Aberystwyth is a bit further, maybe 40 minutes.
Facilities: there's the wood burner, an outdoor patio area, and parking on site. Bedding is available for an extra charge. No pets.
Useful links for planning: Visit Wales and National Trust Wales.