On your way to the Aquinnah Cliffs and Gay Head Lighthouse on the Vineyard's southwest corner, stop for fried clams at The Bite in Menemsha. This pint-sized fishing village of seafood shanties is one of the best spots to see the sunset, as well as pick up the catch of the day.
The farmers' market in the restored Grange Hall (Wednesdays and Saturdays, from 9:00 a.m. to noon) is the place to mingle with locals and pick up a few locally-grown provisions for your cookouts. You'll find stalls selling fresh produce, canned or bottled items like herb-infused oils and vinegars, and even cut flowers and potted plants.
If you want to play in the sand, the marshy beach at Long Point Wildlife Refuge limits visitors, so it feels more secluded ($3 per person June 15 to September 15, free the rest of the year). The surf tends to be a bit rougher than on the other side of the island, but allow the crashing waves to lull you into a state of relaxation. That's likely why you came to the Vineyard in the first place.
One question remains: Can you visit Martha's Vineyard for under $500? Yes. Book an affordable vacation rental and share it with friends. Although my home cost $1,500, it was big enough for four, bringing the price down to $375 each. Also, travel on foot or bike as much as possible, and cook your own seafood rather than dining out. Try grilling clams until the shells open, then lightly steam them in a pot with butter, garlic, and fresh herbs—simple, inexpensive, and delicious. As long as you catch the boat, you won't spend much on having fun.