The RB67 is a very popular camera and is an extremely capable camera as well, it is however the size and weight of a small ocean going liner.
Ash has suggested a Fuji, a very good suggestion, I have had 2 but didn't get on with them (a GW690 and GW690 Mk III), the lenses are absolutley stunning but I found the rangefinder patch small and on anything other than infinity difficult to focus correctly. This however is my fault not the cameras.
This was taken with the Mark 1 version
and this with the Mark III
This is the Mark III
Light and comparitively small and makes massive 6x9 negatives.
There are many, many different types of medium format camera. My own preference is for a TLR (Twin Lens Reflex) which shoots 6x6 square negatives (and we all know its hip to be square) and prices run from very cheap e.g Lubitel (poor quality in my opinion) to very expensive e.g Rolleiflex f2.8 (probably the best ever TLR). If you want interchangeable lenses in a TLR Mamiya is the way to go, C3, 33, 330 or 2, 22, 220 with a range of lenses from 55mm to 250mm.
One from a Mamiya C330f
and one of the camera, they are a fairly hefty beast but not as big as an RB67
Ross Ensign made a series of compact, lighter folding cameras, the 16-20 (6 x 4.5 negs), 12-20 (6 x 6 negs) and 820 (6 x 9 negs) which had excellent glass and a solid build in a fairly compact package. the 16-20 is not much bigger when folded than a larger comapct camera. Prices range from £30 (16-20) to £100 (820), with any with autorange in the name pushing the prices up massively.
Here's the 16-20, completely manual with no double exposure stop.
There are many other choices and I'm sure you will get lots of other suggestions as well as the above.
Cheers
Andy