If you're adding a seating area, maybe you could put some shade-tolerant plants in a few planters under the tree. The problem with planting right under a pine tree is that the needles can acidify the soil, and the roots crowd out anything else you try to grow directly in the soil.

Azalyas love acid soil and partial sun...however they also like quite a bit of water the first year while they get established....if you can get water out there and plant in the fall they may be just the ticket. Be sure and look into the reblooming variety...

Planting under the pine tree is going to be a challenge mostly because the roots from your pine tree are going to grab available moisture. I suggest you do a soil test first because it really is a commonly held fallacy that pine needles acidify the soil. Containers are certainly a way to go if you can get water out there. I have seen azaleas do spectacularly in large containers, which could nicely define your seating area as well.