Belgium is a small country between, clockwise from the North, the Netherlands, Germany, Luxembourg and France. It is a member state of the European Union as well as the Schengen Agreement. The country is divided into a Dutch speaking part in the North (Flanders), and a French speaking part in the South (Wallonie) and a tiny German speaking part in the East. Brussels, the country and European capital, has a bilingual status. In Flanders most people will be able to help you out easily in English. In French speaking Belgium English is quite a bit less commonly spoken, but on the whole people will be able to help you out (especially younger people).
Public transport is extensive and affordable to a certain point.
Trains are expensive, but there's a good deal to be had. For 76 euros if your 26+ and for 51 euro if you're 26-, you can get a Rail Pass (it's called: GO PASS), which gives you 10 rides from any train station to any other train station in Belgium. You can share this ticket with several people. You can also use this ticket for your traject inside Belgium when traveling abroad but you need to write down the last station before the border (even if your train does not stop there), which means you'll have to buy a ticket from that last station to your destination station abroad. For example, when going from Brussels to Amsterdam (and not taking the Thalys) you can buy a ticket from Essen (B) to Amsterdam and write down "Brussel" and "Essen" on your Rail Pass.
The Belgian train (SNCB/NMBS Company reaches all major cities with a regular service. On weekends (from Friday after 19h until Sunday evening), the return ticket is reduced by 50% which is good option for a weekend evasion within the country. The Go-Pass/Rail-Pass is a 10 journeys ticket that allows you to go everywhere in the country (borders excluded). The Go-Pass cost EUR 51 (so EUR 5.10/journey) and is for the -26, although if you don't look too old you probably won't be asked to prove your age. A Go-Pass 1 costs EUR 6 for a single journey between 2 Belgian station. The Rail-Pass, for those over 26, costs EUR 76. The Go-Pass is a good option to go from one side of the country to another. The pass is valid for a year from the date of purchase.
In Flanders (north), the public transport company is DeLijn which completes the train services into the rural towns and inside the urbanised areas. It is possible to buy a 'LijnKaart' which saves you on the cost of local transport. Tickets from ticket machine or in shops give you 25% discount. The current price of a Lijnkaart is EUR 14.00. Once validated you can change buses and trams for up to one hour.
In Brussels, public transport are provided by the STIB/MIVB. There again you can buy a 5 or 10 journeys ticket. Note that buying your ticket inside the bus/tram costs you more. More details will be added regarding Public Transport in Brussels on the city page itself.
In Wallonia (south), the regional bus company is TEC
Hitchhiking is quite doable. Check Hitchwiki.
European Carpooling (Taxi Stop - Eurostop)
Taxi Stop is an agency that offers carpooling possibilities to drivers in destination or from European countries as well as passengers looking for a destination. The website is available in English (as opposed to its German equivalent) but requires to create a free subscription account in matter to contact a driver or a passenger. Although the availability and destination listing does not require to log in. This service is called eurostop and has a set rule regarding the fees which is pretty simple: registration to the service is free of charge, a passenger is simply asked to pay 3€/100km to the driver once at destination. The concept of carpooling is so popular that people might come to you when you hitchhike and offer you a lift. Always clearly say in the beginning that you don't want to pay (assuming you don't) so that there are no misunderstandings.
Hotels are expensive, but you can easily find hosts through hospitality exchange. Free camping is quite hard in the Flandres. In de south of Belgium (the Belgian Ardennes and 'De Hoge Venen') are more accessible for wildcamping. All land is mostly private domain, but it's easy to not be seen.
Bewelcome link http://www.bewelcome.org/places/Belgium/BE
Supermarkets are slightly more expensive than in the Netherlands and Germany. Aldi and Liddle are the cheap shops. There are also a lot of bio organic and nature food shops. (Bruges, Ghent, Antwerp, Brussels...)
Belgium is famous for its frietkoten. These are small restaurants or shacks, that sell French fries (a Belgian dish mind you! Protip: Make sure you don't call them French fries, but just fries. Because some will lecture you about fries not being french.) and all sorts of snacks for relatively low prices. It's not very healthy, but for EUR 5 you can have quite a big meal which is bigger & cheaper then McD's. Every village has a frietkot, and every city has at least one in every neighborhood.
Also make sure you try out the beer. Belgians have the most and best types of beer in the world, and often it'll be cheaper then water or soda.
You can find relatively cheap prepaid SIM cards with data.
Mobile Vikings is one of the best deals for data. 15 euros gets you 2 GB for 1 month, 1000 SMS'es, 1 hour/day free calling to other MVs and you can still use the 15 euro for calling. Internet abroad in the EU is 15 cent per MB.
Belgium can be really good for busking. Part of euro zone and quite wealthy country, the coins you get are usually worthy enough to be saved as an income (traveling cheap with no money for accomodation trasport and food).
All touristic cities downtowns are excellent places and usually police don't bother you too much.