We booked 6 days in Nice for late August, after rightfully guessing that the odds of us ever going to the beach in Portugal were very low, to replenish our sunshine intake before the German winter. What we didn't expect was BF getting a bit sick just before the holidays and that we wouldn't be able to just sit at the beach all day and do nothing, so instead we took advantage of being able to go to any town around Nice for 1.5€ by bus and visited Èze, St. Paul de Vence and Cannes.
The first two are little medieval villages in the mountains that are very worth visiting just to look at them, but the fact that they're filled with small amazing boutiques and art galleries didn't hurt. If you ever have the chance spend a day just getting lost in them.
About Nice itself, I can't say whether it is worth going to as a beach-only holiday, although I loved the little time we spent in the pebbles (I should mention I don't really like sand, so pebbles are great for me), especially in the morning when it was really quiet and the light was great. You should know it's not super hot, there's plenty of wind, very big waves all the time, and jellyfish on occasion. But there are sandy beaches in probably more sheltered places like Saint Jean Cap Ferrat, so it's not like you won't find anything in the area, if you can say no to that amazing looking place in the photo (perspective note: there's a high and very vertical step, about a meter high, between you and the water, that the waves easily reach, that isn't noticeable in the photos).
As a place to visit tho, it's very worth it. There's plenty to do and see in the city and in the towns around it, which as I mentioned before are fairly easy and cheap to reach, the food is great and affordable (the non-vegetarian type, at least, but the markets had tons of delicious looking vegetables of all kinds that I should've brought back to Germany...), and everyone was very nice, especially our host and neighbours (we rented a room through airbnb, you can send a message for details). Pictured above is a big park that always had steam or water jets and was crowded with kids playing in them and adults sitting around it looking jealous (maybe just me).
Neither of us is much interested in the jet-set life, so we just walked around old town for a while, went to the beach (this one has sand and no waves, bf is that kind of person), then back to Nice.
Saint Paul de Vence
You should definitely visit. Every single street looks like a postcard (if you're a bit more skilled than me and you camera is a bit more alive), and there are art galleries, nice restaurants and vintage, antiques and handcrafts shops everywhere. We had lunch at la Hostellerie Les Remparts, and highly recommend (not very budget friendly tho, and book in advance, we were lucky someone didn't show up). I even saw somewhere a Mondrian bag that made me want to start a collection...
Did I mention you can walk to the airport just by going along the beach? You can. We did. Please learn from our mistake.