Trips I've been on:
Big Buddha, about 20 kilometers from Hua Hin. Temple with a giant Buddha. In the temple you can get insight into the monks' lives, and you can see how Thai people act to their religion.
100 % wheelchair suitable.

Bridge on the River Kwai. The famous bridge in Kantanaburi which was bombed during 2nd World War. I was on train trip across the bridge and out through the jungle and mountains. A trip that gave a little to think about, when the railway was built by war prisoners  and forced work, and there died more than 100,000 people in the inhuman working conditions they had in the jungle, with over 40 degree humid heat. At the museum in Kantanaburi we could see everything about the railway construction, and a lot of war effects from 2nd world war

Kantanaburi city and museum is available for both manual and electric wheelchair. However, wheelchair user with the train over the river Kwai bridge is a little cumbersome if you cannot use your legs at all, because the doors are very narrow. Are you not too heavy or wide on the bum they may lift you aboard. The trains door opening  is only about 45 cm, so this trip is not for big and heavy people… I weigh about 72 kg and 176 tall, and there was 5 Thai men to carry me in to the train, it was not a pleasure. But it was a beautiful train ride.

Floating Market. In a large area with rivers and canals  there is Thai floating market, where they sell everything from their small boats. I was lifted into a boat and we sailed a nice ride.
A great experience that is accessible to people in manual wheelchairs. Electric wheelchairs cannot be lifted aboard. But you can then experience the atmosphere from the riverbank.

Monkey Mountain. A few kilometers south of Hua Hin is a small village on the mountain top, where it is teeming with monkeys. They are really cheeky and they're not afraid to "steal" from street vendors stalls or from the tourist’s handbags.
When they get too cheeky the local residents try to keep them away by shooting pebbles at them with a slingshot.
100 % available for both manual and electric wheelchair.

Elephant show. Also in the area of Hua Hin. You can come out and ride on the elephant in the wild. And they have a show where they show a lot of tricks elephants have learned. Including run on 3 wheel bike, play soccer, basketball and bowling, paint real paintings and much more.
100 % available for both manual and electric wheelchair.

Night market, and local marked. Hua Hin has a large night market where you can buy all sorts of stuff for cheap money. There are also many good restaurants. There is a really good atmosphere and nice to just sit and look at people. You can make a good deal here, but some products can be pirate copies. In Thailand you can buy pirated copies to almost everything. Clock’s  and things like that is always copy product  on Thai marked.  So beware not pay too much, and be sure you can bring it legal to your home country.
100 % available for both manual and electric wheelchair.

There are also a big local Thai marked in Hua Hin (ask a taxi driver or some local people where you find it).
Personally I prefer the local Thai market. It is not as touristy, and it is much cheaper to shop there. They have everything, clothes, electronics, fresh fruits and vegetables, fresh meat. The place is open every Tuesday from afternoon to midnight.
100 % available for both manual and electric wheelchair.
 

Sam Roi Yot. National Park 40-50 km south of Hua Hin. Very nice place with many mountains and temples. In the valleys there are shrimp and shellfish farms as far as the eye can see. There are some beautiful beaches too.
You can sail out to some small islands or out to a place with caves and grottoes.
Sailing cruises are cumbersome. But otherwise, Sam Roi Yot also possible to see as a wheelchair user.
 

Bangkok is crowded with people a lot of traffic. I have not yet seen much of the city. But have visited the Royal Palace and the associated large and impressive temple area. Here's temple buildings covered with gold and precious stones.
When visiting here you must remember to be dressed in clothing that covers legs and shoulders, and show respect for the king.
Bangkok is not the very best place in wheelchair, because most places you have to drive on the road, and the traffic is dangerous. So If I’m in Bangkok for a few days, I always go around with taxi. It’s very cheap with “TaxiMeter” taxi.
100% available for both manual and electric wheelchair. However, it’s not possible to enter the temples with a wheelchair.

Cha Am. Cozy tourist and fishing town 20 km north of Hua Hin. Here are a several miles long beach promenade with small hotels, restaurants and resort. Cha Am have many Thai tourist, specially people from Bangkok come here on weekend and holidays.
If you like fish and seafood restaurants Cha Am have great selection.
The town, fishing port and beach promenade 100% available for both manual and electric wheelchair.

Jail House. Danish restaurant in the center of Hua Hin. Should you need some great Danish food, then I recommend visiting Kurt in '"Jail House ". He also sells fresh made rye bread (black bread) and “Danish liver pate” out of the house.
Here are a few steps. But the owner is nice to help the wheelchair inside.

Thai boxing (Muay Thai)
In the center of Hua Hin is a boxing arena to host matches each week. It is a very aggressive sport, where both punches and kicks are used.
There are very loud music, shouting and screaming during the game. Thai people are very happy money playing and you will see the way that money changing hands.
There are stairs outside, but the staff are very strong athletes is friendly  to assist wheelchair users.