Daytrips from Bath/Bristol

Yikes.. nothing? :)

I would suggest a tour trip for that day 2 portion but Bath and Stonehenge would be a full day for sure. Day 3, maybe a tour that originates out of Bath or Bristol that would take you north to Stratford-Upon-Avon and maybe a few other towns.

There are a number of interesting sights in Bristol (SS Great Britain, harbourside, museums) so you should allow time for those. Bath is obviously close and easy to get to by public transport, but really deserves a day on its own. I doubt if you could get to Stonehenge (via Salisbury) as well as see Bath in a single day. Cardiff has its places of interest (Cardiff Bay, Cardiff Castle, St Fagan's Folk Museum), but most people would not think it a priority place to visit. The best castle in travelling range is probably Chepstow. You might then also be able to get to Tintern Abbey by bus from Chepstow, but it depends how the times work out. Most other castles are in more remote places, and need a car to reach. Close to Bristol is Tyntesfield, a large Victorian house built by guano millionaires with its own chapel. A very attractive small city (it has a cathedral) accessible by bus from Bristol is Wells. Cornwall is too far from Bristol to visit sensibly for the day, and the railway stations are not near the most attractive scenery. It might be possible to get to the Eden Project, if that's your kind of thing. The Cotswolds are not easily visited from Bristol by public transport, and places like the Brecon Beacons, which are actually visible form the city, are inaccessible except by car. You will need to look up all these places yourself. They all have websites which will have details of access by public transport.

We did spend some time in Cardiff and enjoyed it. I agree with the suggestions Bob made on what to see there. We also enjoyed the museum very much. When we were there, an Indian festival was going on at the waterfront. We took a water taxi to get there, and it was very nice. Stayed in a great B&B (Town House) and we could just walk from it through the park to get to all the places we wanted to visit. We did have a car to drive to St. Fagans.

If you're going to Stonehenge, you'll want to have a look at Salisbury while you're there, it's a lovely old town. You could also get to Winchester by train, takes about an hour and a half each way.
Or how about Haye-on-wye? Old town, full of book stores, just hope I spelled it correctly!

Excellent suggestions all. Thank you.   I'm thinking that for that final day, we should be able to train to Cardiff for a bit, then train out to Chepstow and bus up to Tintern Abbey. Then head back to Bristol. Does anyone see this as not doable in a day?   @Maggie - I like the idea of Salisbury, however I'm not sure about the logistics. Since we won't have a car, we're likely going to have to take a tour from Bath. I cannot seem to find any that include more than just Stonehenge, at least not with more than the one stop.  
Thanks again and any more suggestions welcome!

It would be possible to visit Cardiff and Tintern on the same day, but you would have limited time in Cardiff, and it would be a long day, with lots of waiting while changing between buses and trains. You can check details at Traveline (www.traveline.info), using the Welsh section of the site. A visit to Stonehenge from Bristol would be best done by taking the train from Bristol to Salisbury, and then the bus from Salisbury to Stonehenge. Us the South-West England section of the Traveline website to check times. Total journey time is about 2 1/2 hours, but you should stop in Salisbury to see the cathedral. There are regular trains back from Salisbury until late evening.