The answer, of course, is trading. You can trade away your top card, for instance. Or you can acquire that type of bean from other players to kickstart a field so you can plant it. The two cards you turn up each turn? They can be traded away too. If you have a problem in Bohnanza, the solution is always to negotiate a trade. Restrictions, in fact, create trade opportunities by design.
Trading games aren't usually fun. Advanced Civilization, for instance, tends toward cruelty since the other person is almost always trying to foist a calamity your way. Pit is chaotic without much strategy. Catan is . . . how can I put this? If you have the right group, trading can be fun. On the other hand, since you can trade with the bank, trading can be biased against, say, the winning player. Plus resource imbalance can put one or more players out of the trade market altogether.
Bohnanza avoids so many of these problems with its mechanics. It's unusual for someone to be completely out of cards because you draw three random cards each turn. Plus you turn up two cards to plant or trade. But trade isn't chaotic since the active player must be involved and must trade with one player at a time. So it's the active player who is in control of the negotiations. If you don't have what they want, you'll sit out that trade round.
Each bean type has it's own payout schedule so common cards earn fewer coins and vice versa. Bean cards have coins on the back, which makes a clever scoring system. It also adds a strategic element since cards that are scored don't come back in future shuffles. Counting cards isn't strictly necessary, but it can make a difference to keep track of where the rare cards are in the game. By keeping score in a pile of cards rather than on a counter or whatnot, there's usually no blackball-the-leader problem in trading.
Now it may be that luck determines outcome most of the time. But I've observed that prolific traders tend to come out on top. You'll sometimes see a trade that is horribly lopsided. Coffee (the least valuable bean in the game) for Red (a far more rare bean) straight up is clearly a bad deal. In coin terms, that usually means the recipient of the Red Bean will bank a coin automatically while the extra Coffee Bean often goes to waste. And yet, the "bad deal" might work out better in the long run if the player can pick up a Coffee or two from each player around the table.
We sometimes forget to take out the bean cards you aren't supposed to use for different numbers of players. It's probably an unfair game that way. Still, the trading mechanic is so fun it's not a problem for our group. It's also unusual for anyone to buy a third bean field. That probably means our strategy is suboptimal, but that's fine too.
That isn't to say there's no thinking involved. Working out trades tickles the brain. Even when you decide you aren't going to be able to trade with the active player, you'll still want to watch what other players offer when you can trade with them. Every now and then a more complex trade will occur to you that, if you can pull it off, will make whether you win or lose the game irrelevant. I don't know of any other trading game that gives that feeling.
We purchased the High Bohn Plus expansion set. I've only tried the extra rules once and they're probably not coming out again. I don't think the expansion is bad (I haven't enough experience with it), but it is unnecessary. The variations seem geared toward adding strategic choices similar to the third field card. I can imagine getting bored with the base game, but we haven't come close to that point yet.
According to the box, Bohnanza is for ages 12 and up. I've had good luck with much younger kids. The key is to be old enough to not shuffle the cards in your hand. Trades can be worked out/explained by players who have a deeper understanding of the game. As long as nobody gets upset with "bad trades" (like I explained earlier) it'll be enjoyable for all.
I'm not sure I'd play with fewer than 4 players. Seven, the maximum supported, seems ideal. If you have more than that, this is a game that lends itself to forming teams to play together. (Far younger players can be involved this way too.) It's wonderful to have a game available when you have so many players. Also, trading works best when there are plenty of options.
The art with its silly bean illustrations suggests a childish game. That's not Bohnanza. It is a lighthearted game, but engaging for all ages. You are forced to trade and it's rare anyone feels cheated or left out. Even people (like me) who dislike trading will have a good time with this game.