If you like vising less crowded beach the you must visit gokarna this is really awesome you would have some decent trekking options too.
for mountains no doubt Leh&Ladakh is best.
you can visit north east too to enjoy natural beauty full of water falls, variety of trekking options and mountains too.
For seashore, undoubtedly visit Andaman Islands.... it is a perfect getaway for group of friends...
if u are looking for a road trip go for a all time favourite option leh and ladakh...
if u want to spend time with nature and enjoy the scenic beauty, try Kerala...
happy travelling :):)