Looking for best destinations between St. Davids and Aberstwyth, Wales - also Brecon Beacons.

So, I take it that on day 1, you leave Chepstow say about 9am heading west, stay one night in the west and then return to Chepstow by the end of the day? That is a lot of ground to cover in such a short time - though it can be done but is mainly driving rather walking.
To justify such a long journey, you really need to spend more time out in the west. For example, driving from Chepstow to St.David’s via the fastest route (M4), belting along at 70 mph where possible, would take at least 3 to 4 hours with no stops to see anything. Then, the following day, you would turn around to do the whole thing again.

So, you could do to shorter day trips from Chepstow or perhaps spend one night out - maybe in the Brecon area?

I would suggest the following as a day trip:- use this map link for helping find the route (colours are live traffic flows);>
https://www.google.co.uk/maps/@51.6872545,-2.7710908,23573m/data=!3m1!1e3!5m1!1e1

Leave Chepstow going north on A466. Can you see the village of St.Arvans? Just after St.Arvans - on a bend of the A466 - a narrow road goes off to the left to Wycliffe. After 2 minutes, you come to a car park on the right. Park & take the track up the hill for about 17 minutes. Where it levels off, you will find the Eagle’s Nest Viewpoint to your right - not clearly marked & it looks like you will walk off a cliff until you see the steps and railings! Now return to car & A466. Proceed to Tintern.
http://cadw.gov.wales/daysout/tinternabbey/?lang=en

Having seen the ruined Abbey - continue up the beautiful Wye Valley on the A466 . Just S of Monmouth - turn R onto A4136 - continue through Staunton and a couple of miles further turn off left onto B4432 and fork left in the village for YAT ROCK. As you approach Yat Raock - follow signs into parking area (pay machines). Toilets available here. Follow trail over footbridge to see spectacular views across the Wye Valley.

Having seen the views, return to car and go in a loop through woods to get back to B4432. Now, you could turn R & head for Monmouth - or turn L and go down the narrow hill under the footbridge - cross the Wye and turn left to reach the A40 expressway for Monmouth. You can visit Monmouth if you wish (ancient bridge at west end of town). Anyway, get back to A40 heading west and turn off for *Raglan and loop back at roundabout to visit castle.http://cadw.gov.wales/daysout/raglancastle/?lang=en
* It is more convenient to see this castle when returning to Chepstow due to the road configuration.

Continue on A40 towards Abergavenny. Just S of town - take A465 going west and about 1 mile turn off left at Llanfoist. Follow signs for B4246 and go up steep hill to Blaenavon and visit the BIG PIT.
https://museum.wales/bigpit/
Here you can go down a disused coal mine (free). Having done this optional detour - return on B4246 to the Usk Valley & head for Gilwern and then Crickhowell where you join the A40 for Brecon. At the village of Bwlch- turn R onto B4260 for Llangorse (Langors) . Suggest you overnight in this area. Just below the village, a dead end road leads to Llangorse Lake. On leaving Llangorse - follow minor roads to Brecon via Llanfihangel.

The tourist information centre in Brecon will have more advice on the area. I would also head for:>http://www.breconbeacons.org/national-park-visitor-centre
The caves are worth seeing:> http://www.showcaves.co.uk
You may like to climb to the top of the Brecon Beacons. Drive to car park just south of summit of A470.

If you just want to do a day trip(s) from Chepstow, also consider a day out in Cardiff. You could go by train.
When going to Manchester - www.nationalrail.co.uk pre book via HFD (change at Newport) as it is cheaper than going via Birmingham.

Thank you James for this wonderful response! Here are the dates we need to fill:

We will have a car and we have reservations so far for:

June 17 - Old Rectory in Tintern (so we can walk and see the Abbey)
June 18 - 19 Canaston Oaks B&B in Narbeth for two nights - any suggestions about what to see? We would like to go out to see the puffins
We have June 20 and June 21 free so far
We must be back in Chepstow on Thursday, June 22 to turn in our car and catch a train to Manchester. We have a hotel in Manchester for the night and fly home on Friday, June 23.

Any suggestions for filling those dates of June 20 and June 21. My traveling companion is very keen on the Brecon Beacons. Is this too far to drive and still get to Chepstow on Thursday, June 22?

Thanks again!

On the Saturday, try and do the Wye Valley stuff. (Don't take walk over iron bridge in Tintern to Devil’s Pulpit - terrible path)!

SUNDAY - drive from Tintern to Pembrokeshire via Brecon Beacons. You need to get to the A40 at Raglan. Put Raglan into satnav in Tintern & odds are it will send you top some narrow back roads. Best to take A466 south to St.Arvans & then hard right (be careful as on bend) to B4293 for Devauden. About 2 miles after village - take ‘sudden’ left turn for Raglan via Llansoy. At RAGLAN visit CASTLE?

Be careful leaving castle to get onto A40 going west for Abergavenny as you direct join going E. So, now you simply follow A40 all the way to Canaston Bridge. (Ignore coal mine & Llangorse detours previously mentioned).

I will now suggest a couple of detours off this route:> from roundabout on W side of Brecon by-pass, take A470 towards Merthyr. After about 4 miles - turn R at Libanus for BRECON BEACONS NATIONAL PARK MOUNTAIN CENTRE. After visit - leave on road that you did not arrive on - pass farm on sharp bend and turn L at T junction. Go along narrow road taking care not to kill the sheep! Soon you arrive at A4215 - turn R for Defynnog and then A40 to Llandovery (worth a look). Then A40 to Llandeilo and second possible detour:>

Take A483 & after crossing river to Ffairfach - turn left for Trapp. Just outside the village is the spectacular ruins of CARREG CENNEN CASTLE. Then return to Ffairfach & take B4300. After about 3 miles - turn R for DRYSLWYN CASTLE (ruin) and possibly also visit nearby ABERGALSNEY GARDENS. Return to B4300 (rather than A40) and head towards Carmarthen. Then A40 to Canaston Bridge
& your accommodation. (Best to follow signs for ‘Oakwood’ as it is not in Narberth - which is a nice little town.)

MONDAY - A4075 to CAREW CASTLE (ruin) - best seen from across the creek. Next - follow signs for Pembroke and take B4319 towards Castlemartin. Turn off for BOSHERSTON and park by the church. Now take the track down to the LILY POOLS. Fab walk along boardwalks & alongside the lily pools to Broad Haven (south) beach. When you reach the coast - turn left (east) & walk along the cliffs - where you may see Puffins & other sea birds. You eventually reach beautiful BARAFUNDLE BAY - which has trees on its western side. You can return to your car at Bosherston.https://www.nationaltrust.org.uk/stackpole

From Bosherston - follow road to ST.GOVANS CHAPEL - which is wedged in the cliffs. (It is possible that the military may stop you using this road or you see a red flag flying which means you cannot walk to the west).

Return to B4319 & head W. After about 3 miles - you see 2 old tanks on L at military base. Turn L for STACK ROCKS & GREEN BRIDGE OF WALES if military permit the road to be open. If you can’t go down this road, return to PEMBROKE to see CASTLE.

If time permits, go to TENBY - which is about a 30 minute drive from Pembroke. (You could consider going to TENBY at the start of the day & the other places after). http://www.visittenby.co.uk

TUESDAY - Take A40 towards Haverfordwest & then A487 to SOLVA (lovely village on an inlet). Continue on A487 to ST.DAVID’S (CATHEDRAL). Consider www.thousandislands.co.uk/boat-trips /
Visit WHITESANDS BAY. Then drive up the coast. (You might like to take the back roads to STUMBLE HEAD). Then head to Fishguard
where ‘Lower Town’ is scenic.
Leave on A487 for NEWPORT (Pemb) . Not far away is PENTRE IFAN BURIAL CHAMBER.
Suggest that you now find B4329 going top over the PRESELLI MOUNTAINS (where the blue stones at Stonehenge came from). Once over summit - take B4313 back to Canaston.

WED. Consider going to TENBY and taking the ferry to CALDY ISLAND. However, you may wish to use this day to start your journey back east - which does not preclude visiting Tenby first but it is unlikely that you would have time for Caldy.
(I am near word limit so will go out & in again).

Driving non stop on the faster southern route (A48/M4) would take about 1 hour 40 minutes from Canaston or Tenby to Cardiff (non stop). Cardiff to Chepstow is about 40 minutes - though jams often happen at Newport. However, we have some interesting places to see en-route, so you might like to book accommodation maybe in the Cardiff area for the Wednesday night?

Just off the A48 about 4 miles E of Carmarthen is https://botanicgarden.wales
Just west of Cardiff - leave M4 at J33 for:>https://museum.wales/stfagans/
(Be careful as it is a sharp turn off the A4232 expressway about 2 miles S of M4 junction. To get back onto M4 - you return to expressway and must loop back from next intersection. Alternatively, stay on A4232 all the way to CARDIFF BAY and see some of CARDIFF. www.visitcardiff.com
http://www.cardiffcastle.com
http://www.mermaidquay.co.uk

M4 J34 in north Cardiff = http://cadw.gov.wales/daysout/castell-coch/?lang=en
and not far away is http://cadw.gov.wales/daysout/caerphilly-castle/?lang=en

So, just some suggestions on how things could pan out.

Disregard what I said about the trains from Chepstow to Manchester as their is hardly any difference in the prices for the 2 routes. Make sure you pre-book specific trains about 11 weeks out to get cheapest prices.

You can click & drag/zoom the map to find the places. (Google the places mentioned for more info).
https://www.google.co.uk/maps/@51.6865797,-4.8458568,23573m/data=!3m1!1e3!5m1!1e1