Try and stay in a hotel on Sukhumvit if you can - it has access to the Skytrain and you can get around the City REALLY quickly. There are LOTS of $30 a night 3 and 4* hotels in town but you will find that Thais like VERY VERY hard beds!
Must do’s
MBK Centre - National Stadium - 6 floors of shopping Warehouse (ladies) and Export / XL for Men are outlet stores (check they aren’t seconds) but stuff is real and cheaper than knock offs. The foodcourt on the top floor is a must for lunch 
Platinum Mall/Pratunam - Platinum is about 1,000 tiny little shops with lots of local designers not a lot for 6 foot blokes but is Mrs D_P’s favourite Mall on the planet - NO Global Brands at all. Pratunam is a maze of old style Thailand retail - Bayoke Tower and Indra Regent Mall - you can spend a very hot day here and come away with a ton of polyester Lacost and Kevin Cleen but we love the place. One BIG point though - you cannot try anything on, so take a Tape Measure! Things like “accessories” that you would buy in Claires in Uk for 5 quid you can buy a suitcase full for 5 quid!
The BEST trip? You can do it yourselves, but take a Map with you from hotel reception
Take the Skytrain to Saphan Taksin station on Silom line. Exit the station there is a Jetty where you can catch River Ferries but go to the Long Tail Boat desks - HAGGLE and book an hour long Canal Trip in a Long Tail (NOT the slow shit things) - when you get off the river a woman “floating market” comes out with expensiver shit to sell you - just buy LOAVES OF BREAD and beer. The Trip goes through the canals - you can pay more to the driver and extend the trip to visit the Royal Barges and the Snake Farm.
The Highlight on the Canals is stopping by one of the Temples and you use the Bread to Feed the Fish - It is insane (link below).
When you exit the Canals, get your driver to stop at Wat Arun - climb it, take pictures sweat a lot. Then get the driver to drop you at Chang Pier. (It is all street food) and walk to the Grand Palace. (Note check the opening hours before you go - think it shuts about 13:00)
Go see the Emerald Buddha and the Temple then carry on to the Grand Palace, have an iced coffee. By now you will likely be exhausted BUT if you have the energy, Wat Po is next door and is a beautiful Temple and also is home to the finest Massage School in Thailand (proper one not dodgy).
Nearby to this area you have Kaosan Road - back packer heaven but honestly not note worthy unless you really are a Di Caprio freak.
That is a very long hot morning but you will see the Temples and the best Architecture. We normally take a Tuk Tuk from here to MBK for a late lunch.
You can book up for “Tours” but you get picked up from your hotel, stuck in traffic, change buses, go to about 5 Temples or the “Floating Market” and taken to a Jewellery Factory…
Most Tourists will get pressured by touts or drivers to go to these places - they get a fee for taking you there - it is interesting but they are designed for ripping tourists off.
Night Time - DON’T fall into the Taxi Driver takes you for Seafood trap. You can eat anywhere. Ditto why go to expensive “Posh” Cocktail Bars - it’s Bangkok ffs.
Nana Plaza is the oldest and most famous “GoGo” area - it has Bar Beers downstairs and you will find as many female tourists as sold old men enjoying the vibe. Tbh the only GoGo bar that has been consistent and “entertaining” has been Angel Witch - the owner is German and his (Ex) Lady Boy wife is a big time choreographer so they put on a decent (and not ping pong) “show” - and yes I let my Kids go there and took my Mum!
The Ping Pong stuff is found across town at Patpong - there is a night market and the main drag. Again, like Nana a high number of LadyBoy bars. Heading down to Patpong you will pass a few of Bazza’s least favourite streets 
The BEST GoGo / Nightlife are for us is Soi Cowboy - close to Asok Skytrain station. we awre heading there for NYE and always have a laugh. You can do the GoGo bit but also eat - Old Dutch Pub does cracking food, and also at the entrance there are two cracking bars - Country Road is one of them which host Thai Cover Bands mostly playing decent Stadium Rock/Heavy Metal classics.
Feeding Fish we do it every time we go to Thailand 