I would also recommend a trip up to Lake Vyrnwy (near Welshpool). There is an RSPB centre there with bird hides and they organise various nature orientated events (such as dawn bird watching etc), there are several cafes, cycle & kayak hire, and various walking trails, picnic sites etc..
If you do decide to go there, you simply must visit the Lake Vyrnwy Hotel (carry on straight past the turn that takes you over the dam, and it is on your right set up in the hillside). When you go up the drive it does look a bit imposing but don't let this put you off, as there is a lovely public bar at the far end of the hotel (opposite to the carpark end). It serves lovely bar meals and you can sit outside (weather permitting) on the patio which has stunning views of the lake. If you are planning to eat there at the weekend , go early as it does get busy.
http://www.lakevyrnwy.com/
Its my personal favourite as I had my wedding reception there.
Also you might like a trip on the Llanfair (Caereinion) to Welshpool light Railway (you can catch a ride at either town).
There is also the National Trust property of Powys Castle near Welshpool -Whilst the Castle interior is not up to much, it has an excellent tea room and great gardens - great for a picnic and they also sell unusual plants.
Near to Welshpool is also the quaint little village of Montgomery - it has a great vegetarian catering cafe, a lovely old fashioned hardware/gift shop called Bunners, nice gallery/craft shop & other gift shops and a (ruined) castle on the hill top makes a pleasant short (15 minute). walk If you are heading back home toward Shrewsbury , definitely put it on your list.