Marriott Soi 2 has an excellent gym...not the best pool for size and I'm struggling to think of a great one in a hotel in lower Sukhumvit....I use one in Phra Khanong called Opus...superb 25m pool and always quiet....the Marriott has a good jacuzzi and sauna however.
For eats I suggest the Italian, Via Emilia...you need to book however....for a superb krapow try Krapow Gapae off Asoke.