Hi Daniel,
I've been a few times but last stayed there in 2006, I think it was. We used it as a base to go down the Mani, further down the peninsula from Stoupa, when our daughter was younger. We have popped in more recently on our travels in August, when it is very busy, but eerily quiet. The beach is sunbed-city in the summer, but with a cathedral like hush. Kalogria beach over the headland is livelier. Stoupa is okay with plenty of tavernas, supermarkets and trinket shops, as well as a pharmacy.
The setting is spectacular with the Taygetos mountains as a backdrop. It is good for walking if it is not too hot, with plenty of villages and Byzantine churches around to explore. There are buses to Kalamata one way and down to Oitylo the other - they are not frequent, but are reliable. There is a fairly easy coastal walk to the next village along, about 5km, where there is a harbour and a few tavernas.
No "far end" opportunities - and none nearby. It is a very conservative area.
I don't feel as relaxed in Stoupa as I do in Plakias, so I think Plakias is more laid back. However I am sure you will enjoy it. Try to get down to the Mani or over to the Messinian peninsula (but that might be too far without an overnight stay - the Kalamata road is a bit scary) - they are our favourite places, even better than Plakias if I dare say that here!
All the best -
Arnaki