So, I didn't want to be one of those people who came to /r/solotravel to get advice, and never post again - But here is what I've done!

I'm 35 Irish (But live in UK) and have come to New Zealand on a working visa (IEP - Similar to Working Holiday, but more expensive and has to be done through a travel company called BUNAC). I hit up Thailand for 20 days en-route and it was amazing.
Wasn't a massive fan of Bangkok. It was busy, smelly and all the locals saw me as a walking wallet. The taxi driver from the airport will scam you. Fact. You gotta insist they put the meter on instead of charging a flat rate, and be prepared to just get out of the car. It can be a bit intimidating, especially as I was fucking tired from being on a plane for so long. Was in Bangkok for about 4 days - Jetlag and general tiredness hit me hard, but meh.

Decided to hit up Koh Tao pretty much straight away, so got on a sleeper train to Chumpon and then on a ferry. Koh Tao only really has 2 things - SCUBA and Drinking. Its a lovely little island, has nice beaches and everything, but its all about diving. Was there for 5 days and did my Open Water and Advanced Open Water scuba courses - Which was really fun. The atmosphere was pretty relaxed, but it was the first time I'd travelled like this in 10 years, so took me a little while to feel confident enough to get out and socialise. Top Tip - Bring sunblock with you. It costs quite a lot once you are on the island.
I then headed to Koh Phangnan for a few days and had my first travel regret. In between Full Moon Parties, the place is dead. Well, the bit I went to was anyway. I'll probably go back there during a FMP to really expereince it properly. Oh, If you are given the option of a Fan room or Aircon always take AC. Its not only hot, its humid.
I then decided to go straight to Chiang Mai as I'd only got a week left in Thailand, and My flight was out of there.
Caught a ferry/bus/flight and was in Chaing Mai. Absolutely loved it. Chinag Mai was what I thought Bangkok was going to be. Temples everywhere, loads of nearby trips, Markets, food... Damn, I already miss it! I stayed at a really nice hostel there (D-Well) It was really social, but not a party place so I made lots of new friends there. Up til then, it had been pretty lonely (Didn't help that I was getting over a breakup, but Hey). When people say 'Take an empty backpack to Thailand, because It's so cheap you'll fill it up' They are not kidding. True, the clothes might not be the best quality, but that's all part of the experience. While there, I did a cooking class which surprisingly was one of my highlights. It was a lot of fun and delicious!

Some of my tips for Thailand - Don't bother Buying a SIM card. Wifi is everywhere, so Whatsapp and other messaging/calling apps are your friend
Always haggle, or work out deals.
The hostel will usually have a 'Travel desk', but its worth going to an independant travel shop to compare what they can offer for trips etc. Also, there are loads of travel shops, so never be afraid of walking away. The street food is delicious, but it can get a bit samey
7-11 does cheese toasties for like 27 baht which is about US$0.80. They are a really easy go-to snack.
Toilets... yeah... So their sewage system isn't designed for toilet paper. Most places have 'Western style' toilets instead of the squat toilets, but they all have a Bum Gun. It's a hose that you use to wash your butt. You then use the TP to dry off and throw it in the bin. Because they know that people will flush TP anyway, the tp they have is... Is there anything less than 1 ply? its SO thin and dissolves when it gets a bit wet. You end up drip-drying more times that you'll admit.
Stuff is more expensive on the islands, but in reality it's only really sunblock and mozzy spray that surprised me.

In that whole 20 days I spent a total of $1200. Of that, about $500 was my diving courses. The rest was accommodation, food, travel, trips, random purchases, drinking etc. Its such a cheap place to be (If you are spending western money)

I'm now in New Zealand, but that's another post entirely! Needless to say, it's all been so much fun. All of the anticipation that built up while saving an planning has not been in vain :)