Beautiful nature, very relaxed area which does not offer a lot of attractions. It's mainly for its landscape (Shan mountains starting there), lakes, river, the hiking opportunities and to wind-down.
Hpa-an is definitely an upcoming nature and eco-tourism destination. However, the city also exhibits a warm, friendly old world charm with it's old fashioned market where traders come to sell their wares as they probably have for decades, if not longer.
Outside of the city, you'll find caves, mountains, even a rock pool you can go swimming in. It's a great place for rest and relaxation, or to get fit cycling or climbing up the mountain. The rock pool, located about 10km outside of town is also a great place to refresh, and if you visit between February and May in particular, it's needed given how hot it is then (temperatures can rise up to 41 or 42 degrees C between March and May and are already in the low to mid thirties by early-mid February). In general, the best time to visit is between late October/November to early/mid February as it's not too hot then (in fact nights can get rather cool) and day time temperatures are warm with dry air; rainfall is sparse. Mid February to May can be unbearably hot with occasional thundershowers, particularly from March/April onwards, while late May/June to early October tends to feature lengthy rainy periods with occasional localized flooding.