Three basic choices: blow it all on one high-risk bet, waste it away in the hope of getting more bonus offers (and keeping the casino happy), or gamble sensibly and face the virtual certainty of some online casinos barring you from bonuses or being barred from play altogether.
An example of the first strategy would be putting it all on one or a few roulette numbers. If you win, play slots until you've met the rollover requirements, then cash out. Few casinos these days allow this and it's an excellent way to be barred from playing there altogether. Furthermore, this is usually a losing proposition unless you know of several casinos which allow it (and even then it's quite risky, there's the gambling risk as well as the risk of not being paid).
Sensible gambling would be playing a game with relatively low variance and near-100% longterm returns: some blackjack and video poker games, a few craps bets, most baccarat games. But the rules and payouts vary from casino to casino--some blackjack games are heavily in favor of the house, and a few video poker games (6/5 Jacks or Better) have poor longterm returns.
No guarantees of course, but with a fair game, sufficiently small wagers, and
flat bets, you will very likely come away with a significant part of the bonus as well as your original deposit. One potential problem with most table games is a $1 minimum bet, $100 may not last very long if you run into a losing streak. (Low variance isn't "no variance".
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And of course many casinos don't count wagers on lower-variance games towards the rollover requirements, or forbid their play entirely. Some casinos even forbid playing video poker with bonus money. (That should be a strong hint as to where their interests lie.)
Your other choice is to waste it away on slots and keno, which is precisely what many of these casinos are hoping for. Then you can throw good money after bad to chase your losses, making the casino even happier and more willing to extend bonuses to you. Lather, rinse, repeat. (It would be easier, safer, and less time-intensive to flush the money down the loo.)
An honest casino won't bar you for gambling sensibly, they know it's up to them to make bonus offers which won't result in excessive losses for them. (Bodog is one of the few who don't offer patently ridiculous reload bonuses and don't forfeit bonuses if they're wagered on certain games.)
Of course you can try some combination of these strategies in the hope of keeping the casino happy while still making a little money. This doesn't work in the long run, very few of the casinos making large bonus offers on a routine basis like to make regular payouts to customers; they'd go broke if they did.