Go to the Gower Peninsula near Swansea, there are loads of great beaches concentrated fairly close together, some big and sandy, some small coves, some with rockpools and some with both. Some easy to access, some not so easy.
Suggestions to start with would be Bracelet Bay (mainly rockpools/stones but next to road and car park, Italian restaurant that is good for kids and and good play area above), Caswell Bay (sandy, next to road and car park, cafe and toilets, good waves), Langland (not been there but supposed to be sandy), Three Cliffs Bay (amazing setting, sandy I think but tricky to get to as I think it can only be accessed by public footpaths or along a long beach), Oxwich Bay (sandy I think, long time since I've been but big beach and close to road), Port Eynon (mix of sand and stones, rockpools, big beach, next to road and close to car park, great fish and chip shop - the middle one in the row of shops overlooking the dunes, ruined building at one end to explore, pub nearby), Rhosilli (very sandy and big beach but loads and loads of steps to get down to it, good for surfing and body boarding), Llangennith (northern end of Rhosilli beach, car park about 5 mins from beach have to walk along path through dunes which gets sandy in parts, massive beach, mainly sandy but dunes behind and possibly rock pools at one end, cafe on campsite where the car park is).
There are some more but I've not been to them and not sure they are so easy to access. You could go to a different beach every day and not leave the Gower, it's great!
More info on this website www.the-gower.com/beaches/beaches.htm - which indicates there are 28 beaches!