Plenty I think, if we're sampling over multiple blocks. Total Nxt transacted, sum of account numbers of everyone who made a transaction, etc. The key thing is that we're taking data over multiple blocks, not just one, so no single forging participant can have large control.
Indeed - as CfB has pointed out "time stamp" isn't going to cut it so I think something like a hash of the sum of account numbers will probably have to do the trick.
I guess also that if the number of blocks required to be known about in the future is big enough (maybe something like 100 might be a more reasonable choice) then hopefully the uncertainty of at least a few forgers not "taking their turn" would make the chance of a forger being able to gain an advantage by omitting some transactions fairly slim (as they would have to had to purchase their own "lottery ticket" exactly x blocks before).
EDIT: Perhaps 64 will be the magic number. I am thinking we take 1 bit (say mod 2 of the second hash) from each of the 64 blocks that come after the block that the tx comes from and then finally hash that. In this way there is no benefit to "mine" for a better hash as each forger (on their own) can only affect the outcome by 1 bit so you would require collusion from numerous forgers (who are all forging one after another) to have a real chance of being able to "rig" the result.