It depends what you like really. We went to Fethiye on the 2nd Night, was very quite and not as lively as we were hoping. It has a beautiful marina and a very good market. The only downfall was it is a good 15 minute walk to the bars, shops and restaurants from where the Dolmus drops you off. We had to stop and ask someone where we could eat/drink.
Hisaranou is more lively and is door to door bars and restaurants. It has a few marge supermarkets, banks, excursion places and all the shops you could want. We tended to go here most evenings for dinner/drinks. We enjoyed the entertainment in Barumba and we also spent a lot of time in Ata Bar.
We didn't go to Olu Deniz during the evening but we did go there one day to see the famous Blue Lagoon. You have to pay to get in (not much) then pay for your sunbeds etc which is pretty standard. The sea is really warm and lovely but the sand is very stoney and quite hard to walk on. Olu Deniz is very picturesque and beautiful, took plenty of photos here. There are a lot of bars and restaurants here, we had lunch the day we went there but as I said, we didn't go there in the evening.
There are details of excursions in the hotel lobby that they run too.
The dolmus is very easy to catch, it only costs 1.5 lira to Hisaranou and I think 2 Lira to Olu and Fethiye.
Whatever you decide to do or wherever you decide to go you are sure to have a fabulous time.